Best Middle ?

A lot of heroes are good for mid. Ashe, Annie, Veiger even Yi (I go mid with Yi a lot)
If you can target you spells easily then veiger and Annie are good. Veiger can go nuts on any enemy hero if you can continiously last hit minions with his spell (AP will go above the roof) and once built he can nuke anyone's ass.
If you can properly use Yi's alpha strike then Yi can be a brilliant selection as well.
If you got mouse controll issues and want a hero easy to control then go for Ashe