Kingdom Rush series, with its cartoon style, hidden secrets, variety of skills, towers, etc. Also Devs already released Iron Marines with new mechanics, so it also good choise for those gamers, who like TD games :)

GemTD

For me, it's Buster's Tower Defense

It's a genre that got built entirely out of Flash from what i gather.

I suppose on RTS games you're more likely when making towers that take out enemies before they arrive, but for a full TD you build a maze and make them run it all the while taking damage. Though what the stakes are and how it runs can heavily vary.

TD needs something other than pure TD to be fun. Watching all the moving sprites across the screen quickly gets repetitive and dull, with health being the main thing of the enemy that goes up.

Yes it is. Less a maze and more multiple fixed chokepoints where you improve firepower to keep them at bay vs a maze. I've probably put more into that one than any other TD.

Defender's quest, it's okay, recommend play on super easy 1/2 star until you get nearly to the end, then go back and redo the earlier ones to get 2-3 star ratings.

Anomaly... I don't know. I was trying it and ended up doing the first mission twice, and at a point there's a choice left or right, except the choice was no choice at all because the result was the same, only which track you got attacked on. Kinda annoyed me and i never touched it again. I don't mind on-rails but false choices irk me.