NIdalee, Anivia, Lux, Blitzcrank, or Janna?

Janna. She's the hawtest among the list.

Nidalee is getting changed heavily soon, so I wouldn't pick her up. Anivia is a really passive champion for most of the game, you farm up until late and then you win because of insane damage from all that farm gold. Blitzcrank all depends on hitting your Q to be useful, if you miss it you don't have much to contribute. Lux is a very skillshot reliant AP carry who can support as well, if you can hit your full combo your enemy's hp disappears, miss it and you have no damage until its back up. Janna is a peel/disengage support, her kit is focused on keeping her adc alive and making sure people don't get close enough to damage them. She's got no engage practically, unless you are willing to make risky, extremely aggressive plays.