I'd say, either GTA:SA or Halo 2.
It's a tough one, since I loved GTA3 (A LOT) but didn't like Vice City at all, I don't know what to expect from GTA:SA, but from what I've seen I think it'll be the sequel to GTA3 I really wanted. It's hard to guage Halo 2 for me too, cause Halo 1 was good but not amazing. The controls, and basic gameplay were great (which is what made MP great) but SP was lacking. Halo 2 will probably be a lot better in the SP regard, but even then I still like objective based shooters over kill horde of enemies, rinse repeate.
So, like I said either GTA:SA or Halo 2. Actually, looking at what I've read, I'm going to go with GTA:SA.