I'll second the various Kingdom Rush games (origins is the latest). They have a minor in-app payment thing where you can buy new heroes to play, but I've never bothered with it and the games are a lot of fun with the default heroes. They are a sort of a tower defense game, but not like plants vs zombies.
Original Plants vs Zombies is great. The sequel is poor, however.
Dungelot is a fun little rogue-like puzzle game, and the mechanics and controls are perfect for a small device.
Banner Sage- TBH I haven't played this yet, but the PC version was interesting to me, if they didn't ruin it in the conversion this might be another worthwhile game.
Rebuild- Zombie Apocalypse turn-based strategy game, had a lot of fun with this one. It's one of those games were you always just want to take one more turn.