low skill high reward:
Top: Garen, Darius, Pantheon, Shen, Singed
Mid: Evelyn, Malz, Karthus
Jungle: Xin, Hecarim, Nocturne
Bottom: Graves, MF
Support: Taric, Taric
Garen is very easy to play because he has a lot of "derp" gameplay with high speed chasing/escaping and really powerful, specially in the early-mid game. Darius is very easy because his kit and base values seem built from the ground up to end lives even if he buys all tank items.
I say Malz is easy because his ult combo will let him derp his way to a lot of very easy kills in the mid-elo range. Evelyn almost plays herself. Karthus has the most quintessential "skillshot" in the game.
Graves is a brute force ADC without any particular skill or timing required. MF runs fast and has an easy to understand kit as well.
Taric can do a lot for his team with barely knowing how to play at all.