The following AD champions are picked more often mid:
* Talon (High burst + Good gankability)
* Zed (Insane burst + Good gankability)
* Pantheon (Good poke + Good gankability)
* Caitlyn (Great poke + Traps)
* Ezreal (Great poke + mobility)
* Jayce (Great poke + burst)
* Lee Sin (Good poke + burst + gankability)
* Talon (High burst + Good gankability)
* Zed (Insane burst + Good gankability)
* Pantheon (Good poke + Good gankability)
* Caitlyn (Great poke + Traps)
* Ezreal (Great poke + mobility)
* Jayce (Great poke + burst)
* Lee Sin (Good poke + burst + gankability)
The question was what AD champs are viable in mid, and I gave him some names :D
Nasus isn't really AD but he's a strong mid with chalice merc treads spirit visage
Gangplank works too :p
Fiora is pretty decent too
Irelia is pretty good
Riven is awsum
Xin Zhao is pretty strong
And Tryndamere
+ Soraka with spirit of the xs (inside joke)
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But the most viable AD mid is Talon. I've also seen Pantheon, Riven, and Vi in mid, other than the ones Meiyjhe answered.