I use fedora, but it's a personal choice at the end of the day.
There are live cd's so you can take a look at them without installing them.

I'd recommend kubuntu, mainly because it uses kde 4.x and it's easy to install apparently - though I've never used it myself (kind of weird recommending something I've never used I know).

I wouldn't recommend CentOS as a desktop env. as it's not as up to date as the others.

Live CD's are the way to go, try a few distros and see.