Elise, Orianna and for a surprise pick, Karma. Elise is just ridiculously strong as a mid lane, and she transitions so well into the current 'league of warmog's' meta that it isn't even funny (i really love the teams that have ALL WARMOGS and no magic resist when i play elise).
Orianna is a great mid champ, and once you get down how her ball works, she can be ridiculous in terms of damage/utility. A full combo of q/w will do a huge chunk of damage, and if you have something like liandry's, you get to multiply all her damage. and she has awesome voiceovers.
Karma, as a mid, is sleeper OP. She has the biggest heal, and the biggest shield in game, and her slow does amazing damage if you can hit it right. She requires a lot of CDR and mana regen, but once you get her range down and how her slow works (you need to tag a minion and make the other person run through her beam) you can melt people. She's also fun against a lot of the current mid killers, like kha and talon, because she WANTS you in close, because then her whole combo can melt you.