I have seen this question many times and even asked it myself a few times too and recent developments in distributions merit an answer. To know which distribution of Linux you should try, you need to think a little bit about what kind of person you are and what role the target system will play. To simplify things, I'd like to list the main four candidates and provide examples of what they are suitable for, and why:
- Fedora: this project was originally started by Red Hat many years ago and is one of the oldest and most popular distributions around. Fedora contains the relatively stable versions of the latest major packages: Gnome, OpenOffice and others together with a lot of cutting-edge software. It is the distribution that's best used if you're a hobbyist and want to try out new features and packages. Its upgrade process between releases isn't very good (in my experiences so far) and it's documentation leaves a lot to be desired. Maintenance is a pain and I would recommend this only for those who are not easily discouraged and like to fiddle.
- Ubuntu: one of the most recent (to me) distributions and by far one of the best on the market, Ubuntu was developed with the idea of being an accessible Linux - it's easy to install, well-documented (to a point), has easy maintenance and a good upgrade path. It even has support for Nvidia proprietary graphics drivers which you need in order to do anything decent with a computer; on other distributions, you have to fiddle about to download and install them - on Ubuntu, you have a built-in menu option marked 'Proprietary drivers'. This is a great all-round operating system and is ideal for beginners. It's not idiot-proof and it's still a big jump for Windows users, but it's worth a look. We're trialling this at work within our development team for the reasons stated here.
- CentOS: this is the free version of Red Hat Enterprise Linux, a distribution aimed at businesses who need rock-solid performance on a par with commercial Unix. CentOS is distributed for free, contains only the most stable, tried and tested packages and has a five-year release cycle. This is best used in a server environment where stability and performance come second to having the latest software and drivers, although you can use third-party software repositories to run whatever you want. We use this at Redwire and have a rolling programme to replace all of our public-facing servers (which use a variety of operating systems) with CentOS. It's incredibly easy to set up and manage, compared to something like FreeBSD and, if you stick to the standard packages, it won't fall over.
So those are three examples; you may be wondering where OpenSUSE, Mandriva or other distributions are and why they're not listed. The simple answer is: I haven't used them. These suggestions are based solely on my personal experience, not on what's fashionable or what's newest. By all means, choose your own path, but hopefully these notes will give you a gentle push in the right direction.