This is just my opinion as of now on when to use the different towers in the game. I am actually unsure of the efficiency of some upgrades but i'll try to go over the upgrades as I understand them. Some of this carries over into the other difficulties but the prices on the towers are different and therefore changes their cost efficiency.
Dart Monkeys: They shoot fast and are the most accurate of the towers, great for picking of bloons travelling alone that survived your AOE damage. Piercing darts is worth it if it will kill 2 bloons 72% of shots. Long range can be worth it in some situations.
Tack shooters: Shoots every 1.5 seconds I think and can pop 5-7 bloons if placed well (placing them not in a corner is not worth it, you'd be better off buying piercing dart monkeys). Can be inaccurate. The faster shooting speeds up shooting by 50%, and costs a little more than 50% of the tower cost but if well placed it's worth it and it's nice that it's cheap. Longer range meh.
Ice Tower: Thought to be useless, and not used in any runs yet, I could see this tower being useful in runs if you can build an army of mainly boomerangs and give them sonic boom and cannons. The upgrades seem useless as you probably just want to stack several of them after eachother in a line and they're expensive.
Cannon: I hate this tower. I don't understand it at all so take what I say with a grain of salt. Extremely inconsistant. I think it can pop 6 bloons in one shot which is not bad but when they are even a little spread out or come from a slightly off angle they can pop anywhere from 0-3. Need at least 2 or 3 for lead balloons in later rounds. Bigger bombs seems just worse than saving for a new bomb tower although if it does allow it to pop say 12 bloons in one hit on a stack it would be amazing, seems really hard to test this out though. Longer range is cheap so worth it if it lets you hit a new part of the map.
Boomerangs: Expensive with multi shot, 745. Extremely good, when he shoots boomerangs into stacks of bloons it will kill 8 bloons and shoots every second. Like a well placed upgraded tack shooter with longer range. Unlike the bomb tower it can pop black bloons. Inacurrate so you need dart monkeys and maybe bomb towers for cleanup.
Super monkey: The high cost alone makes it unattractive to me. Maybe there's a strat where you sell a bunch of towers to buy super monkey quickly. Obviously good but is it better than 4.8 multishot boomerang monkeys? I doubt it. Also doesn't go well with cannons because the darts won't hit the black bloons hiding behind the bigger ones. I've never bought the laser upgrade but maybe it's good.
