As a number of Special Agents has grown, since BTD5 was released, there are a lot of Agents, which can you choose from in the game. While some of them are very helpful, you can consider some of them useless.
A current results are shown below, however you can't vote there directly, so you must use the following link for the voting:
Vote for your favorite Special Agent
Top ten list feature was removed globally, so the voting is no longer available.