Worthwhile online games for 360?

I am considering an xbox 360, but I am curious if it has many worthy online games?

Hell yes.

First and foremost, there's Halo 2. Even though it was out for the old Xbox, you can't play it on the 360.

In addition:

Gears of War
Madden
Call of Duty 2 and 3
Halo 3 (in about a year)

Classics:
Ultimate Mortal Kombat
Street Fighter II

And much more.

you said can't but im sure you meant can. also rainbow six las vegas should be a good one.

might wanna fix that error about halo 2... you "can" play it on 360

I already have halo 2...

How is the new marval game (obviously not a competition game, but is it good online)?

well whats kinda a downer is that u can only use the characters that the hosts has/unlocked. meaning you couldnt use your character online unless your the host. the arcade mode is pretty good with some friends and some achievements are based on that mode.