Imo, My top 3 are:
3: Fiddlesticks: With all of the bursty people around fiddle dies too fast, but with a correctly played ult he can get a triple or even a pentakill.
2: Nidalee. She has a huge damage output but dies if not played correctly. Her spears are hard to land, especially in lane.
1: Urgot: If played right is the best kiter in the whole game. Takes down tanks and is really tanky but lacks mobility and burst.
Honorable mentions:
Leblanc deals eye-watering damage but dies really fast. If you get your combo off you are going to win your matchup 90% of the time.
Evelynn is hard to get to know, but then becomes really easy. She has trouble building tank and high burst, but can deal crazy sustain damage.
Champions who I think are portrayed as hard, but aren't:
Riven: All you have to do is qqq.
Swain: Has some difficulty but if you get Roa and zhonyas you can win most 1v1/2 with ease.
Katarina: Slam your head on your keyboard and you get a pentakill.