Is karthus the best carry for mid lane in a teamfight?

No.

They simply provide more; either AoE damage or utility.

Is a teamfight beast. Always has and still is. The catch with ori, is that she requires a decent team to follow up on her utility. I'v been doing good with her so far in ranked, and her laning phase is decent, but prohibited with the mana regen nerfs. I reccomend trying her if you haven't, but she is tricky, and will fee clunky at first. Heck she always feels a little clunky xD

I don't think is the best, as he lacks hard CC. He can do a crap ton of damage, but winning team fights is more about CC than damage output.

Those are the best ones I'd say. Are also great because you can mind game your opponents by not letting them know which lane you are going to. Lissandra can go top or mid and do well, and Morgana can play top/mid/support.