Lee Sin might be overwhelming to you but he is easily one of the most fun (in my opinion) and most diverse. He is fairly easy to pick up but hard to master. There are ALOT of combinations you can pull off and he is really a great pick in any comps. Alot of depth to his kit.
Master Yi is an exceptional jungler with very high clear time. He is also very easy to pick up but hard to master his full potential (looking at you cowsep)
Now in terms of success for new players I suggest Varus as an ADC. He is immobile but you will most likely just out damage your opponents anyways (in an even scenario). He is strong early which is where most of the brawling occurs with newer players.
Now in terms of actually learning to play an ADC i would suggest Lucian. He is one of the most mobile ADCs and has great damage if you use your auto attacks correctly. Positioning is a big factor for him as he lacks the long auto attack range other ADCs have.
Now if you're talking raw late game damage output, that hands down goes to tristana. Fairly weak early but has the longest base range at level 18 with a MASSIVE attack speed steroid. Solid carry.
If you want to have trolly fun be Urgot and watch your enemies cry in lane. Really REALLY strong early game but has mana issues.