Depends on your playstyle really. There are ALOT of mid laners out there, but here are some I enjoy playing

Xerath is all about one thing: POKE. His Q has a long range, W is an aoe slow, throwable stun with E, and pretty much airstrike ultimate.
Pros: Poking all day long
cons: skills can easily be sidestepped/juked/dodged

Ziggs, the bomber man XD
ziggs is fun basically because his kit makes him annoying. Though, his q will take some time to get used to, since it bounces and is a unique skill shot on its own. W is a minefield that basically creates sort of a visible teemo shroom where people wouldn't want to step on, thanks to the slow. E is a position displacer, and ur escape if ever an assassin comes at you (also, this is why people hate ziggs in aram. They use this on base and everyone gets knocked out of base before they buy stuff XD). And, your ulti is a big ass freaking bomb of doom that, when your fed, is a panic zone for your enemies (also, epic ks skill)
Pros: annoying as hell
cons: needs skill to use well

Lux, the girl with the big ass lazer
Lux has tons of cc on her. Q is a skillshot snare that locks down up to two side by side targets. W is a shield that you can also give to your team and yourself (which makes her a good support as well). E is a slow bomb . And R is a FREAKING LAZER.
Pros: lots of cc, team player
cons: all spells are skill-shots

Cho'gath, League's version of Godzilla
Cho'gath is a tanky mage that can also be played mid. His Q is a delayed knockup zone which helps in poking, also, applies a slow after knockup. W is a cone silence. E is sort of like your AA has splash but goes in a straight line rather than around you. And his ultimate is NOM NOM NOM NOM, which basically is a true damage chomp fest that, if used to kill minion or enemy champion, will enlarge him up to 6 stacks
pros: You get to be as big as godzilla (sort of)
cons: q is rather hard to get used to, and the right use of R seperates the good from the noob players
Honorable mentions

Anti-melee, all out mage combo

The go to assassin mage for mid

You're a ninja with a shadow by your side so you're never truly alone

The swordsman that stomps everyone when fed thanks to his INSANE crit% and annoying shield

He's an emperor and he orders soldiers to attack for him. Like being bossy, pick this guy.
Yes, there are plenty more mids, but these are those that i would recommend. Not all are easy to use, but all are fun to play as