Easy Mid Laner to buy?

Pretty much always a solid pusher, play safe, get that mana regen going, push forever and always stand behind your front line. Almost never have reason to be anywhere else since you can shove far lanes with your ult on a fairly low CD towards the end of the game.

Akali/Katarina/Malzahar

Veigar is the easiest forget what anyone else says

You have zero risk because you can just sit back and lightly poke early game, and just wait for level 6. Once you get your ult, you can melt anyone below 100% health before they can even get a cast off.