Thinking back to what games over the years I put ALOT of time into... Diablo, Diablo 2, Ultima Online, Starcraft, Warcraft III, Medal of Honor: Allied Assault (the first in the MoH series), World of Warcraft, Company of Heroes (which I have played nearly everyday for well over a year) - all PC games. One of my favorite multi-player games of all time is NBA Hangtime on the 64.
Anyway, if I have to pick one, it is Medal of Honor: Allied Assault (mulitplayer) - PC.