Worms on live is really good.
I also play aegis wing, marble blast, and Sonic 2. I'd play undertow if anyone was interested, but nobody is ever online.
Sonic 2's multiplayer is extremely broken. The host will always win, because ther second player gets a horrible lag.
I'm always up to play Aegis Wing and Marble Blast, but I have to really be in the mood for Ikagura. I can't believe the game is actually harder in multiplayer.
As for single player games, I think Galaga Legions is very under-rated. I really like the game.
I am a cheap gamer, so I love the arcade. Just look at my played games, the majority are arcade titles.
EDIT: Not to just plug Another Soundscape's game, but the XNA title Artoon is actually very good too.