I'd strongly recommend getting the official doom game by ID. The controls are very tight and responsive and it's a game suited to the iPhones size. If you needed something more current, NOVA 2 has the edge in terms of single player, and MC2 has a cracking online multiplayer. Rage is a fair suggestion, but some people aren't too keen on the on-rails game mechanic, if you can live without controlling your movement then you may well like it. Incidentally made by the company behind doom.
BFBC2 is choppy. I don't have NOVA2 but it looks damn good. I do have MC2 though, and it's awesome. BFBC2 lags quite a bit and is limited to 4 players in multiplayer, while both MC2 and NOVA2 have a limit of 10. The only other iPhone multiplayer FPS I've played before these came out was Eliminate Pro, and compared to that MC2 is much smoother (much less lag).
