Halo: Spartan Strike is good. Actually, it's really good. It builds on Halo: Spartan Assault, and save for a few niggles here and there, it does a great job of transposing Halo's frantic gun-fights to mobile.
It might not be an FPS, but there's still something decidedly Halo-y about the experience. Plus it looks gorgeous, and it comes in a decently priced bundle with its sibling.