Which Linux?

"The trouble with choice is you have to make a choice”
A prospective Linux user, who stumbles upon DistroWatch and realizes there are over 500 “distros” or distributions of Linux may be forgiven for running screaming back to Windows (And you thought there were too many versions of Vista! ).
Yet if you are a new user, unless you have special needs or simply a desire to explore the wonderful variety available, you can probably set aside all but a half-dozen or so. I think we can use a few simple criteria to filter the list to a more manageable size:

Ease of installation.
Unless you buy one of the many computers that come pre-installed with Linux from Amazon, Best Buy, Dell, Sears, Walmart, and many smaller companies, you will have to install it yourself, or have someone help you do so. Modern distros are easier to install than Windows, but novices rarely install that either. This shouldn't scare you off if you can follow directions though.

Ease of use.
Many distros are not targeted at newcomers to the Linux world, and will not be a good first choice. Among these are Debian, Fedora, Gentoo and Slackware. All fine distributions, but skip them until you have some experience with Linux (With a new graphical installer, Debian might be the exception here for an adventurous novice).
Even Linus Torvalds himself favors easy to install and use distros.

Major distribution.
You should start with a distro that has a large user base, because there will more likely be better support options, user forums and documentation. In addition, the larger organizations usually are more likely to have the resources to be able to keep the distro up to date and solve problems quickly as they come along. They also tend to have larger package repositories so you have more choices of software to install, and better long range planning and support. Another viewpoint on how distributions are seen by the Linux community (not necessarily by novices).

Freedom
You're looking for something better, right? So don't tie yourself to an OS that has links back to the proprietary model. Distros that I personally avoid on ethical grounds, include SUSE, openSUSE, Linspire, Freespire and Xandros. They all have ties to Microsoft, the constant enemy of innovation, standards and freedom. You can do better.
(See No Microsoft! for a fuller explanation of why you should avoid them)

(Note that I am not talking to the experienced Linux user here, and perhaps not even to the technically oriented Windows "Power User". The last two points bear thought in any case though.)

So which one to use then?

I wish I had a definitive answer, but this question is hotly debated among Linux devotees, and has been for years. Still, if we apply the four criteria above to the top 50 distributions at DistroWatch (by page hit ranking, an admittedly crude measure of popularity), then I think we are left with:

Ubuntu
Linux Mint
PCLinuxOS
Mandriva
Kubuntu
MEPHIS

(Not in order of preference)

Before looking briefly at each of these, lets get a couple other items on the table. Though not deal breakers, I would also consider:

Software installation & upgrade
Although generally very easy today with graphical software installers, I think the Debian-based distros with their deb packages are more highly regarded in this area. Most other popular distros use the RPM package format--PCLinuxOS, Mandriva, Fedora, and many more.

Desktop, or GUIs (Graphical User Interface)
The two main ones are KDE and Gnome. I think most users coming from Windows will like KDE better as it will feel more familiar. It also has a reputation of being much more configurable and customizable. It also has higher rated applications, though you can run either one's applications on the other desktop. Some distros, like Ubuntu, let you easily choose either, even after installation.

OK, you're saying, SO WHICH ONE IS BEST?
Ah, if only life offered such absolutes! All I can do is offer some observations on the seven distros we have selected above:

PCLinuxOS
Nice distro, smaller community than the majors, but many vocal supporters who claim it is THE one for “It just works” functionality. Some say it's popularity is exaggerated. Originally based on Mandriva. Helpful user forum and very good documentation. Glossy monthly magazine with good articles geared to new users.

Ubuntu
Started by multi-millionaire and space tourist Mark Shuttleworth, it has grown from nothing to dominating the top spot on Distrowatch. Based on Debian, uses Gnome desktop. Known for innovation and a new release every six months. Dell is now selling desktop and laptop PCs with Ubuntu, but Ubuntu has lately began to attract criticism. Whether justified, or motivated by jealousy and elitism we'll have to wait and see. It has a large helpful user community, lots of excellent documentation, a long term plan, and commercial support for business.

Linux Mint
A very slick smaller distro, based on Ubuntu and has both Gnome and KDE Desktop editions. It has "lots of desktop improvements and a strong focus on making things work out of the box". Integrated and configured codecs makes playing multimedia easier than most and it has a few special applications to manage updates and software installation, etc. More cutting-edge than Ubuntu with all the pros and cons that implies. Large user forum.

Kubuntu
An official sub-project of Ubuntu, so all the same comments apply, except it uses the KDE Desktop. Although it usually lags behind it's parent in getting the latest features, it is still very usable, and more easily customizable. Like other KDE distros, it uses the excellent KDE applications, like K3B for CD/DVD burning, Amarok for playing music, etc. Also like Ubuntu itself, it offers versions with long term support.

Mandriva
Until Ubuntu came along Mandriva (formerly Mandrake) was at the top of most peoples list of beginner-friendly Linux distros.
Trying to get too commercial, financial difficulties, bad customer service and too many bugs dropped them in the rankings for quite some time. Mandriva is seemingly making a comeback and bears consideration again.

Mephis
Another smaller distro, similar in outlook to PCLinuxOS, but based on Debian, not Mandriva. Has been somewhat commercial in nature, like Mandriva.
Software is not always up-to-date and no obvious long term plan. Nice looking distro with KDE desktop, it retains many fans.

Update Jan. 2009

It seems that gOS (previously included here) has developed a relationship with Microsoft, and is in some sense promoting Windows on it's web site. I would therefore not recommend gOS to anyone who cares about freedom.

Conclusion
So what do I use? Kubuntu since mid 2007 (before that, Gentoo for four years. Starting in 1998 I used Red Hat, Mandriva, Suse, and Gentoo for periods of 6 months to 4 years).

It suits me, and I hope this helps you find one that suites you too.
That, in the end, is the only criteria that counts.