On the other hand, you have Akali who can potentially snowball an entire team into the negative simply with a few early kills. There are a few keynotes though. For one, unlike Mundo who becomes unkillable after getting like half a tank item, Akali is weak with almost no kill potential pre-6. Also, she's going to be useless for majority of the game if she get behind as with most assassins. On the postive side, very few people in low elo actually build the enemy comp. So if you're a 8-0 Akali, rather than buying mr, they'll just keep going with their standard dmg build and you can keep picking them off until you feel like Voyboy.
As for who to pick, I'd go with Mundo as well. Get tanky, run right at the enemy in team fights, and watch them focus you for the next 30 seconds.