Anivia. The bird is the word, son.

Thank you everyone for your opinions, I really appreciate it

Malzahar. I was in the same boat as you. Now hes my #1 played char. Amazing burst and damage. Incredible in lane. Great Ult . Cant ask for anything else

I play Galio and he is great, and he's supposed to be a tank but I play him as a Caster, and sometimes support on the side if I'm not getting enough kills and alot of assists. He does great damage with AP stacked, but he has a pretty steep learning curve so it will take a bit of effort to master him.

I have Anivia and Galio and I play with a friend who has Malzahar..

Malz > Anivia > Galio.

Also, in terms of least difficult to learn/play/dominate with

Malz > Anivia > Galio

Galio has an AMAZING Ultimate and is overall a fun champ to play but his Q W and E abilities are all a bit lackluster on damage. Great for support, though.

Anivia is a solid mid and has a great damage output and her Passives is quite possibly one of the best in the game. Between her stun, slow, wall, and AOE ultimate, she's fairly well rounded.

But...Malz is just a beast.

hmmm omnomnom....Assassin caster?
kog maw Ap can assassinate but skill is need.....
maybe ezreal too
ulting people b-ing at towers from halfway acroos the map :P

Get Galio.
He is, in most of the case, very badly played. So if you can play it well, you can earn a sense of achievement.

Or you can buy the click-r-to-pentakill karthus if you play with non noob teammates who can actually harrass when they are laning.

if you like casterish type nuke/assasins.

Play Akali. One of the highest if not the highest nukes in the game (been a bit since I have checked... but its nasty)

Otherwise Irelia is rugged.

Malzahar is pretty fun, you can harass and farm at the same time without having to lift a finger early, thanks to voidings.
You just spam wall to harass, get your voiding, use visions on a low-hp minion and profit from underfed enemy champs and high gold on yourself.
he also doesn't require you to be a comboer, the 1-2-3-4 combo of e,q,w and r is easy to pull off and does as much as leblanc.
Also his passive is a good way to deal damage while you're in cooldown (hourglass is also good to use in that situation).

Caster + Assassin? Kassassin? =)