This app was mentioned in 12 comments, with an average of 1.42 upvotes
I'll give you a couple
Fieldrunners 2: An old favorite that is still great.
Bloons td5: A large td game that is a staple to the genre.
Bloons Monkey City: An rpgish version of the regular Bloons TD game.
Kingdom Rush: This series of TD games are quite good and polished but the first one was turned free with ads, however the sequels are still a couple of dollars each and make improvements in the game
Fieldrunners 2 is similar https://play.google.com/store/apps/details?id=com.subatomicstudios.fieldrunners2&hl=en .
I'm huge fan of Fieldrunners 2. In my opinion the best tower defense game on play store. You should know, that Kingdom rush Frontiers is on sale at the moment. Go pick it up for a buck just because. Also as mentioned already in this thread, Element TD is surprisingly well made android port of at least decade old tower defense game concept. (there is also free version, but I haven't bothered with that).
/edit: I also remembered TD called Tower defense. It was free and it was awesome. I finished that game without any IAPs on all difficulties. It definitely deserves a shoutout on some list of sorts.