A great game for the iPad is Heavy Gunner 3D by Com2Us.
This is a single player fast action shooter game, where you control a turret with loads of weapons that each has many options and upgrades. This game is rated as one of the top 3D first person shooter games i've seen in a long while. From personal experience, unless you have large hands, it can be difficult playing the game, however it looks just stunning on the iPad screen.
As written from Com2Us website: Features ▪ 3D Mechanic FPS ▪ Campaign Mode & Challenge Mode (Infinite Survival) & Global Online Ranking ▪ Upgradable weapons with various types and levels allowing more than 120 combinations ▪ 35 Missions with 3 levels of difficulty ▪ 50 achievements to be awarded with special medals