I agree with several of the ones already mentioned, but I didn't see Gridrunner. https://itunes.apple.com/gb/app/gridrunner/id502840657?mt=8 There's a free version (standalone, not a "cut down" version of the other version) if you want to try it out. No ads, no IAPs, just pure, hyper-intense shooting goodness.
Orion Nebula is the most colorful SHMUPs https://itunes.apple.com/us/app/orion-nebula/id667474657?mt=8