Top: gangplank- barrels are confusing as well as the gp farming mini game. Ult is pretty bad to beginners because of the map awareness required. Otherwise, top lane is very beginner friendly. Note: ignite> tp
Jungle: Lee sin doesn't even fit into this role, ATM he's just press q twice and press r and the enemy is donezo. I would say fiddlesticks fits the bill as well as nidalee(does she even count anymore?) idk, jungling is a pretty hard role for beginners, might be best to avoid this role entirely.
Mid: azir's hard, Aurelion sol, katarina, leblanc cassiopeia, pretty much anything that isn't Annie lmao. Also would not recommend this role, the plethora of champions are incredibly quirky as well as the amount of game knowledge in order to make it work makes this role as bad as jgl.
Adc: I recommend this role highly, you don't require much game knowledge and the main mechanic you have to learn is csing, which you have to learn anyways. Is the role that will help you get better at league the fastest imo. Champions to avoid include urgot/draven/Kalista/vayne
Sup: mechanically not rigorous, however the shit you have to do in lane makes this actually quite challenging as beginners should not have to be faced with warding and lane control and peeling. Would not recommend this role either
Tl;dr play adc or top lane cz I said so. Source : s4 diamond s5/6 master