Rumble is one of my favorites. Just like Veiger, he has fun lines and a amusing death. He is hard to use, but fun to learn. He uses no mana, but he silences himself for 4 seconds so make sure your next spell is one you want to use with the bonus he'll get while silenced.
Viktor is the only mage I really felt was both OP and underwhelming. Once you learn how to use him, his abilities are sure to get tons of kills. Make sure to upgrade his passive to what you want in late. Most chose the burn for his Q, others may want the bonus mana regen. Depends on you.
Malzahar is hard to learn, and uses mana like crazy. If you get lots of mana regen, you don't even need to worry about AP, his abilities deal tons of damage without it. Once you get AP, his abilities now deal even more damage. Make sure to keep his E ready. It's what he'll use the most, for his passive and mana regen purposes.
Diana is one I'm unsure of. She is fun, and may seem like an assassin, but she's a fighter/mage. So, go crazy. You're main priority is to hit them with your Q. Then, your R will deal bonus damage (like Akili's). Once you master her, she's hard to get away from, and the passive she has is insane.
Nautilus is one I like to use mid, others prefer bot support, jungle, or top. Nautilus is easy to learn and master. You have a nice gap closer, in your Q, that also is followed by a fun W to slow them down. Also, after a Q on a enemy champion (or really whatever) your passive deals (not only bonus damage) but stuns them for a second.
Talon is coming back as a top notch assassin once again. Try and make use of his E. It will help your passive when dealing bonus damage. First E the enemy, W to their position, quickly press Q before you attack to lane a major amount of damage on the enemy in one fierce move.
I don't know what you have right now, so I can only make one other suggestion. I'd suggest you get Tahm Kench. I've played him on the PBE. Don't have him yet on live. He's easy to learn but hard to master. Wait until this Wednesday or water 7 day period after his release to get him at just 6300. I liked playing as him, as others say as well. I was going to say Bard, but you don't seem like that kind of guy. Tahm Kench is allowed up top and jungle, so if you feel like it, try it. You can help allies with your W, and send them places with your R. He's also hard to kill.
Its all up to you now.