Champs played by people who are in the right place at the right time against a team that is not working like a team should.
But champs most prone to get pentas?
I would say Irelia is a great pentakiller because she gets her dash back if she kills the person with it, meaning if timed correctly it can be used to catch up to just about anyone.
Also, Cassiopeia is another one. If she lands her poison on you, she not only gets an MS buff, but she also gets infinite E's on you. Her ult also makes for a great catching/initiating skill.
Katarina is surprisingly good at picking up multikills. If she isn't focused, her ult wrecks people very quickly. Then again, it's incredibly easy to shut down all her damage.
Master Yi is a great pentakiller because if he comes in after a team has already found their target (Like the AP carry) he can just ult in, get a quick kill, get his ult back, get another kill. And basically he can just clean up a team fight and you can't get away from him with just slows.
And finally, Olaf is a great pentakiller because he has 6 seconds of "You can't stop me even if you wanted to"