Garen- not cheap, but really easy to play with, high attack, high armor, strong ulti.
Warwick- easy,quite cheap, to pwn with you need to have 3-5 round experience.
Master Yi- very easy, very cheap, but as someone said at lvl 30 playing as Yi is hard as hell, I agree with that, since I've tryed not long time ago.
Ashe- a cheap pwner champ, but to pwn you must have experience with her and know an ultimate built, also tank-dependable, cuz she's really squishy.
Mordekaiser- a pwn char tank, very rare at ~10lvl games, but really often at ~30lvl games, can be built as dps, tank or nuke. I prefer tank built, since with other builts he's quite a squishy guy.
Malphite- my favorite support champ, also easy to pwn with, I remember when I've tried him, I had 16/0 k/d ratio, it was on ~13lvl game and I've never seen that champ until I've bought him, so it makes him the most rarest champ to see in game. ^^