Of all the ADCs I've played during free rotations, if I had to learn any it would be a toss up between Ezreal and Vayne. Ezreal was really fun to play in an AI game and has decent mobility and watching the ulti kill someone from the other side of the map is priceless. Vayne's ability to reposition and utterly stomp on people with the ultimate if you get good at her (as shown in the LCS) on top of the really powerful stun/knockback from Condemn also made her great fun to play.