I'm totally loving Ahri, I love Kat aswell. What are some other fantastic mid picks? I was thinking of buying Cass next, or maybe LeBlanc!

Don't buy LB, I can tell you that much.

I really like malzahar safe pick for me and can farm very well with his E.

Honestly any of the common mid picks are just fine for solo Queue. It's not about the champion, it's about how you play the lane. Learn the lane (no one in low elo grabs wraiths, for instance - assuming an equal skill matchup this gives you a 65 gold advantage every minute over your opponent, in other words it's like adding an extra 3 minions every other wave) for solo queue. It's hands-down the best way you are going to do well in solo queue. Or ranked teams.

Watch some pro games where they go mid. Notice how they harass, when they push, etc. Also pay attention to how often they grab wraiths. They roam as well. Try to implement these tactics and no matter what champion(s) you play, your Elo will improve.

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.

I did my climb out of hell with Swain who is awesome and you won't go wrong with him but I wish I had put my time into mastering someone a little more team fight friendly. For that I recommend Zyra. Even if you're completely shut down from farming, which is impossible because she is an amazing farmer, she is fantastic in team fights.

There is no real best mid.
Just buy something you like, someone viable and learn that champion to each and every detail you see on him it's on your mind.
I do Ahri mid most of the time because i learn how to counter most mids with her.
LB is great, but you can only do so much stuff and you better do it before lvl 12, once there...nevermore.

The all time under rated ziggs. WRECKS team fights. And he has next to no counters mid lane

Morgana and Gragas are fairly solid picks for mid... Both are hard to learn but pretty powerful and easy mode once you do learn them...