Ahri would likely be the most mobile because of her ult. Fizz also has the ability to bounce around a bit. Kassadin's Ult is a blink (as apposed to Ahri's dash) and has a much further range than Ahri, it can drain your mana VERY fast though.
For people new to mid I always recommend Annie. Her Q is free when used to last hit minions,, and she gets a free stun every 4th spell cast. She is also a very strong ganker because of the stun and instant cast on her ult, or the single targeting of Q. She however has no escapes except for a possible well timed stun. So take that into consideration. High Burst, Good trading, Great ganks, very easy to learn. She is average in terms of overall stats, many others have higher damage, a few others are beafier, many others have better utility. However, her kit is forgiving to somebody learning APC.