While I love Orianna, I vote Lux or Anivia. They both have pretty unstoppable farm, good damage, utility, and are nearly impossible to gank.

Karthus. He has a decent escape or killing tool with his wall, his passive mana regen from farming gives him massive sustain, and he has a global ult to get kills with. His passive is one of the most frightening in team fights. He can safely harass and farm with his Q.

Depends on your criteria. Mine is ease of farming, effectiveness given good play by the other team, viable transitions if behind and ability to snowball if ahead would put Gragas, Oriana and Anivia all as about even and all the best picks you can have mid.

Heimerdinger.

Katarina is best in Low Elo. Ultra Pubstomping.

Zyra is a bit op at the moment and a good Morgana can win lane against just about anyone but...

Personally my favorites to play are Galio (Amazing counter/damage after you pick up a chalice), Orianna (if you've watched her played well before you know), Eve (that burst!), and Karma (underrated but a ton of fun).

Cass, Kass, Ahri,and Morgana are all good. Personally, Cass does very well in lane. Morgana may not be as good but she keeps her game throughout the game. Ahri is good but seriously, depend on how you can land does QWE. Kass, really depends on who you are fighting. He is very tough and a good choice for mid.