Yeah Rainbow six black arrow for the xbox kept me occupied for so long.
Halo 2 is good online, apart from the ongoing fights between american and british players
Yes halo 2 awsome online but it really all depends on your interests if you like FPS's, MMO's, RTS... it really depends on your tastes in games for which you think if the best online played game.
RTS:
-C&C Tiberian Sun
-C&C Tiberian Sun: Firestorm
-C&C Red Alert 2
-C&C Red Alert: Yuri's Revenge
-C&C Generals
-C&C Generals: Zero Hour
-Warcraft II
-Warcraft III
-Starcraft
-Starcraft: Brood War