I don't think there is a best mid... Some champs just counter others and etc. Every AP mid champ is pretty good imo. The only ones I don't really like nowadays is like Swain, Vlad, Teemo. I prefer to go top with 'em.
Morgana is probably the best safe pick imo..Pretty easy to learn and master. Though if you suck at skillshotting, you probably will not be good on her. She can farm easy, escape easy, I really love her ult too so you can tell if a stealth champ is trying to gank you or whatnot. :-)
LB is pretty decent in terms of 1v1'ing mid lane, but later on when people build MR, you'll feel pretty worthless. I think she doesn't bring enough to the table to be a safe pick.