Don't forget SLAX (one of the Slackware's sons),
if you just want it in your favorite USB key it's really easy to install and customize as well. I've never used it heavily for years since few months ago, I've had few projects with live distros and live filesystems in RAM and I've started with it again. It's really easy to install/configure if you're using an USB stick.

For servers and HA systems I always keep source based distros (when time is an option) or I prefer long term distributions where stability is a must

I preffer Ubuntu 10.10 Maverick Meerkat, it works with my video card drivers and i've had the best luck using wine with it as well.

The very best Linux distro is the one that:

1 - Works best on your hardware, and

2 - Does what you want and need it to do.

Other things to consider: When you choose a distro, you are also choosing...

1 - ...repositories! Some are little and out of date and offer very little to safely install on your 'puter. Some are ginormous and awesome!

2- ...a community! Ubuntu, for example, has this hyoooge, awesome, friendly, busy community of ordinary kids like me mixed in with some amazing Linux gurus with mad geeky techno skills that love to help us ordinary users out! Some other communities are barely there, or maybe they're kinda snobby and elitist, and some don't like kids. Their loss.

I love Xubuntu! It's a well-kept secret, even in the Ubuntu community, overshadowed by the new KDE and Unity stuff. But the latest version is sleek, pretty, simple enough even for me, lol, and fast even on my nearly 8-year-old Dell! The latest version is free of all that Gnome cruft that weighed down most of the earlier versions. Wonderful awesomeness!