Kassadin, Akali, and Annie are good gankers
Annie is an extremely effective ganker and has low cooldowns. Her ability to kill opposing champions is completely reliant on your ability to combo your spells and use stun procs to your advantage.
Akali relies on jungling to be a good ganker, if that's what floats your boat, if you're a lower level, it's probably not a good idea to jungle Akali.
Kassadin can close gaps quickly between him and his target, and has a strong escape, making him a reliable ganker
Amumu Rammus and Malphite have reliable mid game burst combos and ganking ability but should fill their role as tanks late game.
Nidalee, Pantheon, Twitch, and Evelynn have strong "combo" and ganking abilities.
However, Nidalee has a high skill ceiling and learning curve, people will hate you for playing Evelynn. Pantheon has poor late game scaling, and Twitch is expensive, but you should keep him in mind, especially if that's not a problem for you.