Which towers should a beginner lean on in bloons td 6?

Glue Gunner helps more than people think for slowing ceramics and moabs later. And put your Alchemist upgrades on the 4-0-0 path when you can afford it, Stronger Stimulant buffs your big hitters a lot.

Ninja Monkey is the one I'd learn first, it sees camo once you upgrade it and does well basically everywhere. Pair it with an Alchemist next to your main damage towers and a Monkey Village later for camo detection and range. For the early rounds a Dart Monkey or a Sub (on water maps) is plenty. Spike Factory near the exit is a nice safety net for leaks. Biggest beginner mistake is spreading cash over ten cheap towers instead of pushing two or three up to tier 4.