Modern Combat 2 is debatably the best FPS on the iOS. It has an excellent campaign, and it isn't rare to see the multiplayer server count at 3,000+ at different times of the day.
Ok... it might be silly but I've been hung up on Doomsday lately... also HacX has the same design is free. Those are all worthy.
Mission Europa should appear on the app store any day now. Infinity Project is another (more Half-Life-esque) FPS that I enjoyed.
Which is more popular ? (as in has more online players?) http://appshopper.com/games/modern-combat-2-black-pegasus-hd or http://appshopper.com/games/modern-combat-sandstorm-hd ? Rixx
Modern combat is hands down the best FPS on iDevices with the best graphics ( until warm gun comes out) it has about 2000 players at all times compared to N.O.V.A 2's 300 something. N.O.V.A 2 came out during same week as Infinity Blade & Real Racing 2 so it wasn't as praised as either of the games & it copied halo to the max.