I don't think the Vez'nan tower on the last level counts, so:
5.) Shadow Archers
Shadow Archers seem terrible on the first couple of levels because you don't really have enough gold to max it out. Once it is maxed out, it one of the best towers. Not only does it have a fast fire rate, it can wreck almost wreck any enemy. This is like the Blazing Gem except it doesn't cost money.
4.) Spectors Mausoleum
Spectors Mausoleum does decent damage, and it's a good tower to have at a choke point, because it can spawn Gargoyles (Which I find extremely OP because they have 300 health). Besides, this tower costs very little gold to build and upgrade.
3.) Elite Harassers
At first I thought I needed a good barrack tower so I wanted to buy the Grim Cemetery. But then, I realized the best barrack in the game was right here, unlocked on stage 13!
It spawns the best soldiers ever! I thought it was going to be a sucky version of the Twilight Harassers from Origins, but it turns out to be better than some heroes! This tower was very helpful on the level City of Lozagon.
2.) Orc Shaman
The Elder Shaman sitting on tower ravages every enemy that dares to come into its wrath. The only reason it doesn't sit on the #1 throne right now is because it doesn't spawn anything as one of its abilities.
1.) Rotten Forest
I don't know if its really powerful or not, but every once in a while I have a huge pile of dead enemies with their head on the ground near this tower.
0.) Blazing Gem
I won't really explain this one, but every enemy that comes near this tower dies in like 10 seconds-including High Sorcerers. It shoots an inferno that annihilates every single enemy. This is just way better than Wicked Sisters in my opinion.