the only ones I can remember seeing are an urgot (just now) a rengar (I got in bots), a **** load of ap yis and katerinas, two nasus (one of which being mine) an ashe (funny as hell), a caitlyn, a ryze, and darius (quite a few there too, freaking down right) with maybe an evelynn if memory serves me well.
But just based on observation I would say Kat is the easiest. Example, the game I saw urgot get one just now, in the 25 or so minutes I watched, kat had two quads. her clean up potential is unmatched. I argue she has more than yi because yi if you get unlucky enough to have your ult end during an alpha strike well bad luck for you. Even in cleaning up assists only do half (again if I remember correctly and its kinda sad I don't know this because I'm beginning to main ap yi, for normals anyway.) For kat its her passive. You get one, you reset. Game, set, match.
I forgot vayne and zyra too for my list of people ive seen. and there were a few vaynes too. Also Even though I agree its possible to penta with anyone, I challenge someone to get a penta with leona.