As a newcomer to the Xbox (I still love my Wii, but...there's just not enough titles to satisfy me, and some of those 360 games just looked too damn fun), I was wondering what games some of you might recommend for having the best online play? I obviously have Halo 3 and think it is fantastic.

A friend told me Rainbow 6 had fun online play, but I don't know how much I trust his judgement. Thanks in advance!