Always Ahri. She's good for just about any situation in mid. Pokes, CC, mobility, teamfights, sustain...she's not the best at any one of those things, but she does all of them pretty well. Makes her a good "Mario" choice, when you're not sure what you want your mid to do.
If you're good at landing combos (with Akali and Kat, I'd assume so) Swain is a great pick. He has ungodly sustain, his damage is deceptively strong, and he's got plenty of CC and teamfight presence. Plus, he can also run top. Gives you another option there.
Morgana is always solid. Impossible to dislodge, good sustain, teamfighting...the only issue I have with her is that her combo feels sluggish and weak. I guess she's more of a matter of taste. I'd keep her in mind. Likewise Lux, only with less sustain and more CC.