Nasus teaches you defensive play, lane control, and last hitting

Shen/Twisted Fate will help you with map awareness.

Varus teaches a lot about adc positioning. As he has no escape and no permakite - you need to put yourself where you can disengage with his AoE or bottleneck divers with his ult.

Olaf and Zed! very difficult for people who are trying to learn, but once you pick them up and start going really well with them, they teach you about positioning, map awareness, kiting, skill-shot landing etc.
another good one would be wukong teaches you how to juke(can really get you out of a tough situation, but then again so can zed) and if you REALLY want to be taught a harsh lesson, play trynd and see just how important cc and focus is

fiddle

Ill say it again.
Sivir, her E teaches you how to focus on the enemy, and react to their moves.
Her Q teaches you how to harass.
Her R teaches you when to pop ults, and the importance of saving them for team fights.

Ezreal and Shaco, learn to 1v5 by kiting, baiting, abusing walls, stringing out the enemy team and bursting them in isolation or picking them apart with skillshots.

bump