Based on community opinion, most GNU/Linux distros fall into the first, second, or third tier. Noting that a distro is popular or influential is worthwhile, but knowing the reasons for its popularity is better still.
At first, ranking GNU/Linux distributions seems alien to the spirit of free software. After all, free software is all about choice. What should matter is that your distro suits you, not how others judge it.
Yet, in practice, community members judge distributions all the time. They don't use a single metric, and at times a distro's appeal is as simple as the fact that it is new or has released a new version. Yet, whenever community members choose a distribution to download or to build their own distribution upon, or to borrow a tool from, they are making a verdict on it.
The exact position on the Page Hit Ranking on the front page of Distrowatch may change, but, if you ignore the new distributions, over several years, a reasonably consistent picture emerges of how distributions are ranked relative to each other.
Looking at Distrowatch and daily news stories, I suggest that many distributions that have existed for more than a couple of years tend to fall into four main tiers. On the first tier are about half a dozen that have the attention of the majority of users. In fact, for some users, especially new ones, one or more first tier distros could be GNU/Linux, for all they know.
On the second tier are distributions that attract a dedicated, but small following. And on the third tier, those that, while not necessarily inferior to those on higher tiers, occupy specialized niches and hold little in popular appeal.
Finally, there are the rest: Distros too new to have found their place, and those that, for one reason or the other, are unlikely to attract much attention.
First tier distros: The Giants
At any given time, about half a dozen distributions are the most widely used and influential. Few GNU/Linux users will not have at least one of these distros installed on a machine somewhere at home or work. They are the ones that are most often mentioned in the media, and the ones that other distros are derived from. Most -- but not all -- have both a strong commercial and community face, and all have been in existence at least eight years
Occasionally, a first tier distro may slide -- it's hard to believe now, for instance, that TurboLinux was a distro to watch in 1999 -- but the attitude towards first tier distros remains remarkably stable. Their positions may change on Distrowatch's list, but most of them remain consistently in the top ten.
Debian: In the last couple of years, Debian has slipped a few positions at Distrowatch, partly because of the rise of Ubuntu. However, Debian retains a respectable following. If it is now valued less for itself these days, no other distribution comes close to it in inspiring spinoffs. In fact, of the top ten at Distrowatch, six are currently based on it, and some of these now have spinoffs of their own.
Fedora / Red Hat Enterprise Linux (RHEL): Fedora is the community version of the code that eventually becomes Red Hat Enterprise Linux. Both are the modern descendants of Red Hat, which was probably the number one distribution between about 1996 and 2000. The reorganization caused the two modern distros to sag in popularity in the earlier part of the millennium, but both have recovered nicely in recent years. Fedora is known for introducing new programs into the greater community, while RHEL is a stable and successful commercial distribution.
Mandriva: Formerly Mandrake, Mandriva ruled the desktop in 2000-01. However, financial troubles, buggy releases, and some bad publicity over the firing of founder Gael Duval has caused it to slip until it just barely belongs in the first tier at all. Despite all these troubles, Mandriva continues to be one of the most innovative desktop distributions.
Slackware: A hardcore geek's distribution dedicated to high performance, Slackware could not claim a first tier position on popularity alone. However, its influence on other distros remains as strong as ever, with only Debian claiming more derivatives.
SUSE Linux Enterprise / openSuSE: openSuSE is the community version of SUSE Linux Enterprise. Survivors from the Nineties, the SUSE distros have suffered from the fallout of being bought by Novell, and then Novell's infamous agreement with Microsoft in November 2006. openSUSE, in particular, never seems to have developed the close community that its technical quality deserves. However, SUSE Linux Enterprise remains one of the most successful commercial distributions, and both distros include unique features such as the YAST configuration tool and the SLED menu for GNOME.
Ubuntu: In less than four years, Ubuntu has come out of nowhere to become the dominant desktop distribution. In the Linux Foundation's Client/Desktop Survey for 2007, its various incarnations accounted for 55% of desktops. Part of this success is due to its building upon Debian, but it is also probably the most innovative distribution today, especially when it comes to usability.
Second tier distros: Challengers
Most second tier distros are more unstably positioned than first tier ones. Some distributions in this category are strong contenders for the first tier, but, though they might eventually get there, are still finding their own level. Others are long-established, and have attracted stable, medium-sized communities without being the sources of innovation or spinoffs that their first tier counterparts have. In addition, the challengers are less likely than the giants to be equally successful in developing both a business and a community.
CentOS: Based on the Red Hat Enterprise Linux source code, CentOS could have become first-tier, except for the long delay of its last major release. CentOS's appeal lies mainly in its compiling of Red Hat source code into freely available binaries and its reputation for testing, since its versions go through three levels of testing -- Fedora's, RHEL's, and its own. In the four years of its existence, it has hovered consistently around number 15 in the Distrowatch list.
Linspire / Freespire: Freespire is the community version of Linspire. Linspire has attracted considerable attention, but much of the attention has been of the wrong sort, including outrageous statements by CEOs like Michael Robertson and Kevin Carmody, and questioning of its technical decisions, such as the relaxation of the distinction between root and everyday accounts. Nor has its proprietary CNR repository proved a hit, though some hardware bundling deals seem to have established a small Linspire community. As for Freespire, it is too recent to have attracted much notice.
Linux Mint: Based on Ubuntu, Mint attracted considerable attention on its first release, mainly because of its emphasis on desktop usability. Each new release continues to attract much the same attention. But in between releases, its popularity slips considerably, suggesting that its a distribution that users love to try, but that comparatively few stay with.
MEPIS: A few years ago, when Debian had the reputation of being difficult to install, distributions like MEPIS emerged to provide the Debian experience with an easier install. This need no longer exists, but MEPIS still retains its partisans.
PCLinuxOS: Of all the second tier distributions, PCLinuxOS is currently the one most likely to become first tier. In the last six months, it has consistently topped the Distrowatch download list, thanks mainly to a user-friendly desktop that borrows from most of the first-tier distributions, but particularly Mandriva and Debian/Ubuntu. However, whether it can sustain this recent popularity remains to be seen.
Xandros: Xandros is the descendant of Corel Linux, which in 2000 was one of the first-tier distributions. But, despite an emphasis on servers in the last couple of years, Xandros has been unable to capture the imagination of most of the community.
Third Tier Distros: Petty Officers
Like petty officers on a ship, the distributions in this category represent a parallel hierarchy. In other words, while there is a continuity in the rankings of the first and second tier, those in the third tier have settled down to become prominent in their own niches. Unlike those in the first and second tier, these distributions make no attempt to offer something for everyone; instead, they are designed for very specific purposes. For this reason, distros in this category are judged less by popularity or influence than by how useful they are for particular purposes.
The truth is, there are too many specialized distros to list them all. However, here are some of the better known ones:
Damn Small Linux (DSL): One of the first and best-known distributions for small or minimal systems, DSL has developed a community that is unusually loyal for free software. Among its peculiarities is a continued use of the 2.4 kernel and a resolution to limit the size to 50 megabytes.
Foresight: Designed for those who want the cutting edge of GNOME and of cross-desktop tools, Foresight is one of the few distributions to use the Conary packaging system, which combines modern packaging techniques with version control. Although Conary has so far failed to overtake RPMs and DEBs, many of those who have used it regard Conary as the future of package management.
K12LTSP: As the mouthful of a name suggests, K12LTSP is doubly a specialist: It is an educational distro (K-12) designed for use on thin clients (LTSP). Within this space, it has few rivals.
Knoppix: One of the many Debian-derived distros, Knoppix was the first live CD to capture the community's attention. Although it is far from being alone today, it remains many user's choice of rescue disks.
Puppy Linux: Like DSL, Puppy Linux is a minimalist distribution. Built from scratch rather than derived from another distro, it runs off a RAM disk and uses smaller window managers like Fluxbox and IceWM for a graphical interface.
The limits of ranking
Nothing in this division suggests technical or philosophical superiority -- just popularity and influence. Rather than an exact taxonomy, these rankings should be thought of as an attempt to summarize the reactions to various distributions. So far as possible, I have tried to observe why each distribution is regarded in the way that it is rather than voice my own views, although undoubtedly my own bias has crept into some of the descriptions.
Nor have I tried to categorize every distribution known to me. For one thing, I haven't tried every known distribution, and would have no wish to devote the rest of my life to doing so.
For another, aside from the specializations that clearly fall into the third tier, beyond the three categories I have mentioned, rankings become harder to describe. For every distribution like Ubuntu that quickly establishes its position at the head of the pack, dozens more exist, whose popularity is more like that of a penny mining stock, changing dramatically from year to year.
Many, such as Zenwalk or Dreamlinux are too recent for their position to be clear for another year or two, while others never seem to have been intended as anything more than a two- or three-person project.
And the ranks within each tier? That is something I have no intention of getting into. Although how each distribution is regarded broadly speaking is relatively easy to observe, beyond that, personal experience and enthusiasm create too much static for consistently clear signals.
All the same, placing distributions into general categories can be useful if you're browsing distros, especially if you are new. Noting that a distro is popular or influential is worthwhile, but knowing the reasons for its popularity or how it has influenced is better still. If nothing else, this knowledge can save you time by allowing you to target what interests you.