The only online game ive ever really played is wonderland online. Quite enjoyed it apart from the mass of nasty players.

Live for speed

If you are talking about general computer games then I would have to say resistance 2 because you have the choice or co-op or general multiplayer which is equally awesome. There are not many games where you can have 8 player co-op (not that I have played anyway)

Farmville Great game!!..

M favorite game is prince of persia

NFS is the best game for me.