I don’t have a favorite yet, but I’ve tried.
Knoppix was cool because it got me interested. Fedora was cool because I learned a lot trying to get things to work . Mepis is nice because it boots and installs off of the same CD and seems to have the software an average user would want. Ubuntu seems like it’ll be pretty cool (I’m going to install over Mepis now, maybe I’ll get a dual boot thing rigged this time round), but the thing I like about it the most is the whole idea behind the project (see website).
Knoppix didn’t seem very usable (never took the time to figure out how to do a hard drive install). Fedora was troublesome, but I’m sure it makes for a good server. Mepis and Ubuntu feel really nice.
One nicer thing about Mepis is that it comes with Divx drivers, the ability to play MP3s and stuff like that straight off the CD. Ubuntu doesn’t as to not get into legal trouble.
There’s my fity cent.