I started with MkLinux. This was a mistake. Lucky it's a mistake you probably couldn't repeat if you tried really hard. If you're actually amazingly talented enough to make it work in this day and age, it probably wouldn't end up being a mistake, since you'd have to fix everything mistaken about it in the process.
I then tried RedHat before there was a bunch of "We offer RedHat" distributions. Because back then, RedHat offered RedHat with or without cost, which was identical to all the other RedHats people used. This actually didn't go so well. As in, it wasn't worth the effort. I then briefly tried Yellow Dog Linux but noticed it was scarily similar.
I then tried Debian. This was wonderful. It was also the only Debian based system available at the time. I would now recommend Ubuntu for a wonderful first-time linux experience.
Then I started working with Solaris. And I would recommend it for anyone. It was around this time I got sick or rebuilding the Linux kernel just to keep up with the latest packages that for some reason wanted to use the latest glibc. And I wasn't on the bleeding edge.
At the same time I decided to use a bsd. I tried the three major ones available and found NetBSD supported the most of my hardware and was actually a tad better documented then. This actually produced my longest uptime. What I liked best about the system at the time was that it was actually very simple by staying complex. Does that make sense? For the version I used there are no extra tools needed to administer a NetBSD box beyond sh and ed. You'll switch those to bash and vi in the process, but you'll never have to start X11 or run any kind of control panel or settings application, you won't have to restart after installing anything, there's only one right way to do each step, and that way is learnable, repeatable, and exact. Building the kernel is not an exercise in pruning. Configuring the file-system has no path-less-travelled (Rieser!). The same can be said of network setup (on the software side, assuming you don't like token ring) and running standard daemons.
As some point I tried a Knoppix and a Gnoppix. Neither managed to do the promised Myth stuff that they both said they were specifically built for.
Then I got wind of Gentoo and thought "I like NetBSD pkgsrc ports, so I might like portage." This actually convinced me that both portage and pkgsrc are seriously flawed in conception. It's just not a good idea to have OS developers spending all their time patching other people's software so it'll build and run on their OS. This is because 1/4 of the time something won't work the way it's documented that it should, or some option will be unavailable or some interdependency is impossible to fulfill while various maintainers are out of sync. Also building stuff this way, and Linux from scratch, are just a lot of wasted time. If you want to learn this stuff, pick up an older NetBSD release (3?) and a book.
Then I tried OpenSuse 10 just to see if anyone had actually approached the install process of a commercial product. I dare-say they have, as have Ubuntu and others now.
Currently I'm happy with Mac OS X. Yes. Really. My linux use is relegated to damn small linux and SystemRescueCD.