Best overall gameplay: Halo 3. It had an even playing ground, great maps, great mechanics, great weapons. Unfortunately it's kinda dead. I picked up a used copy of ODST for $10 which includes all the maps online earlier so I'm hoping a few thousand people still play at least. Definitely the most competitive oriented shooter out there for mainstream gamers aside from like CS or something.
Best buy atm: MW3 or BF3. MW3 is a great game, not very balanced though. Definitely the best COD I've played so far, I just wish they would have added in certain things from blops like wager matches and whatnot. BF3 is just fun as fuckin hell.