What is the best thing to buy in btd4

Best map ever is monkey temple - build 40 towers or more and get at least 30,000 dollars. Then it will ask you to sacrifice that much and sun god temple will activate. It will kill almost anything instantly. Not a hack. You should regain the money in a couple of rounds. Temple is 5x as good as sun god.

Upgrade super monkey until you reach sun god for the best tower.

Next best would be monkey apprentice. Upgrade until you get sorceress that makes lightning tornadoes.