Any assassin with resets (yi, kha, katarina) and any adc are your best bets.
And darius.

In all honesty why do you want a penta? Its fun and all but think about it do you want to win game or just completely ks your team and make some noobs rage.Also make you rage for getting stopped at a quadra. Leaving my vayne, talon, katarina, ap yi, and brand quadra aside, you should focus in team fights on the carries not just OH I HOPE FOR A PENTA!!!!!!!! But those who really want a penta all i can say is its not impossible but remember that everyone wants kills these days.

easy, well thats relative, but id say the top 5 easiest (for me anyway) katarina, vayne, fiddlesticks, miss fortune, and karthus, now granted some need prefect situations to get pentas, like fiddle ult during a 5v5 at baron, or karthus cleanup
hardest, janna, taric, karma, maiokai, and shen, i think those are harder due to lack of abality to spam damage, and low damage in general, better get a friend to spoon feed you

pentasa are hard tho man, ive gotten more quads than i can remember but only like 2 actual pentas non bot games
those were with darius and katarina, although i should have had one with zyra but i was like a second too late on the 5th kill
darius pentas arent that easy unless you build full ad, in which case its still hard as yo uwill be focused, also most people arent stupid enough to let a darius ult their entire team, for pentas, you need mobility, they will run

My very first penta was when Brand first came out. All of that AoE was ridiculous. Then, my second one came with Fizz. Again, the AoE ult and his troll poll allowed me to not be touched at all.

Singed is easy. Just find 5 stupid people and run around at low health.
Its also proven that Singed can more easily get Penta Kills when the Benny Hill Theme is playing in the backround. Its a fact.

How to get a penta with janna: ARAM PG,

Vayne

Depends on what champions you play most of the time, support and tank pentas are extremely uncommon, where ADC/Assassin/Bruiser pentas can come off like no one's buisness in certain cases.

Kill resets and very high mobility/burst make it a whole lot easier in general though, in that case I would recommend Katarina, Akali, Tristana, and Kha'Zix. Happy hunting, and remember that mass CC/range champs also do quite well, such as Varus/Lux/Karthus/Ashe.

Vayne is also good, just because if you can get to mid/late game in good shape you now have 3 steroids and all the items to support them along with high mobility and epic jukage.

That being said, my only pentas are on Ashe (first real game penta) Zyra (second, all in one ult, only our Taric was really there helping, but considering he baited them all, he helped a lot) and Vayne, just because late game ADCs with strong kiting/chasing do that.