Kopete all the way...
Software selection is always a matter of opinion, of course, but I have never understood why GAIM always wins these polls. I don't like it. Never have. It feels very clumsy and awkward to me, especially since I use KDE in favor of GNOME.
My choice has always been Kopete. I think it beats all the IM clients on Linux, as well as those for Windows and Mac OS-X. IMHO, it is very clean, well organized and has all the features I want.