Sky Force is freemium heavy, but has great graphics. If you are looking for a premium experience though with similar (if not better graphics), go for Air Attack 2. AA2 is bloody hard (especially with the bomb mechanic) and is downright impossible if you have trouble multitasking, since missions require you to bomb the ground and take out airplanes at the same time, and often the bombing objectives move around as crazy as the airplanes do but you have to time your bombs more carefully. If you want something non-traditional, Liberation Maiden is fantastic. Fantastic. It's a premium app, so you get everything when you buy it, and it has the best story of any shmup on the App Store. Plus, you get to pilot a giant flying robot. Want old-school? R-Type, R-Type II, and Raiden Legacy are all available but are hardcore (especially the R-Type games, which basically give you 4 lives). But they are amazing console quality experiences (gameplay-wise) and they are dirt cheap. Abyss Attack is amazing freemium shmup/runner hybrid which has kick-ass graphics combined with a very unique take on the genre (you are a submarine killing aquatic horrors). You can also unlock the stages by doing particular missions (or just buying them once you beat the boss) and the bosses look amazing. Plasma Sky is another "runner-style" shmup which has really smooth graphics and interesting encounters but is also exceptionally difficult and a bit bothersome if you don't like playing the exact same encounters over and over again. For arena-style shmups, the best out there are Geometry Wars 3, Hyperlight, and Pew Pew. Honorable mentions go to Noiz2sa Free and rRootage Online. Both are Kenta Cho masterpieces but like R-Type, they play much better with a keyboard or joystick than a finger because invariably your finger gets in the way and gets you killed. They were designed for whole screen experiences. But rRootage is unique in that it is a boss shmup, where you just fight bosses. I really hope at some point somebody ports over Warning Forever, because that is one of the best boss shmups in existence. If you haven't updated to iOS 9 then there is a great Indonesian shmup called Inheritage. I'm not sure if it's still on the App Store but it is one of the most amazing shmups on iOS and deserves an update.
