What is the top secret item in bloons tower defence 3

Buy the Monkey Beacon and keep upgrading it until you have Monkey storm in the place of top secret. It costs 850 each time you use it but it clears everything.. even the big balloon surf board thingys that contain LOADS of balloons.
Hope that helped!
p.s: It's good to buy this when you are at least level 30 as everything is very expensive!