My opinion. ranked by usefulness:

S tier:
* Gatling
* Anti-Air
These two are pretty self-explanatory. Gatling is cheap, cost effective and puffs up soakers, which is the one thing the player weapons are really bad at doing. Anti-Air destroys most flying waves with minimal investment, even on higher difficulties

A tier:
* Accelerator
* Lightning
These two are the anti-heavy towers, so they're almost as important as the S-tier ones. In terms of absolute power the Accelerator is by far the strongest tower in the game, but if you only build them you're going to have a bad time. Lightning is weaker against single targets but better against waves of medium-high strength enemies. Its DPS is also entirely frontloaded, so it's great on straightways that chargers are going to zip down.

B+ tier:
* Scatter laser
* Slow field
* Holo
I didn't know where to put these towers, so I made a special tier for them. I find the best use for scatter lasers is to combine them with Anti-Air towers so they can pick off weakened spore pods. On higher difficulties they're not that good against ground targets compared to more specialised towers, but their ability to hit both air and ground is fantastic on maps like Aftermath where you'll die if you don't have some of their hybrid damage.
I mostly use slow fields in combination with Accelerators to maximise the amount of time they have on targets, but they're also good when combined with Holos so you can splash more targets with Holo-boosted grenades or Tesla.

B tier:
* Violator
Violator gets its own tier. I didn't like them much at first, but I've warmed up to them. They're great at killing flying enemies of all kinds, but they're especially good at killing the birdlike ones that I can't remember the name of. I still don't like them that much against ground targets, but they CAN hit them which helps with their cost efficiency.

C tier:
* Kairos
* Penetrator
I rarely build these, but the Kairos is great, or even almost necessary on a couple of Insane maps. The reason I don't have it in B with the Violator is because it's usually overkill. Penetrators are very powerful but I have a hard time convincing myself to use them instead of other towers. They're also slightly bugged and occasionally miss targets for no reason.

D tier:
* Drone
* Mortar
* Killing floor
There are uses for all of these, but I almost never build them. Mortars are pretty good on Complex, and I have used Drones on Cavern before, but you can always do better with other towers. Killing floors would be in E tier if it wasn't for Invasion, where the enemies spawn in very dense groups. You can do a lot of cost effective damage with a killing floor there, so they do have at least one use.

E tier:
* Amp field
I think it's worse than every other option, unless you're just trying to see how big you can make your sniper numbers. I guess if you freeze a huge enemy on top of it you can get a lot of damage out of your towers, but I'm pretty sure a slowfield gives more overall effective DPS.