I used Trillian for a while but it's almost too much; it runs a big file on the PC, it has more options than I'd ever think about using and, more than anything, the interface actually gets less intuitive as you use it.
I can easily say that GAIM is the best I've used yet; I can be logged onto AOL, Yahoo and MSN at the same time, it takes up less resources than Trillian and, most of all, it's much more intuitive.
Now, if only more people would use GoogleTalk....
