Short answer: Go with the Rocket, Magic, and Patrol Towers.
Long answer: It depends on how you define "powerful." Are you looking at pure attack strength, or factoring in range and attack speed as well? One of the special towers (I believe it's called the Knight Tower) gains power while it's inactive, letting it hit the hardest after storing a long charge, but it lags behind in action-heavy areas. If I had to recommend a single unit, I'd go with the Rocket Tower, considering its speed, splash damage and range.