I'm partial to Fedora, mainly since I've been running flavors of RedHat all the way back to .99 Alpha, and have just kinda' grown with it.
The only thing that pisses me off about it is that each new rev comes with some new "neat" utility packages that do nothing but bulk the system up. I prefer to edit just about every config in their respective textfiles, or script it across machines. I don't want a few gig default installation, but that's what I've been getting of late. I think that kinda' sucks.
Slackware is a close second for me, but it seems that Slackware systems require just a bit more attention and tweak-time, which can become an issue when you're running farms of bunches of servers on mixed hardware. Fedora seems to play more uniformly across the board from quad Xeon procs to old PIIs. Just my personal observation.
When it's all mostly GNU based, it just becomes a matter of preference... Sort of like Windows, and do you like the newer XP
start menu / desktop, or do you do what I do and instantly switch to "classic view".
I've heard that Ubuntu is pretty cool, but I haven't had the time to install it on anything. You also have your SuSE fanclubs, etc.
So all in all, no real recommendations, just a few comments on my experiences. The best recommendation is to go with what you're comfortable with and is stable enough for what you want it to do. Gee, I'm really helpful.
We can agree to disagree.