mine was with master yi :D i basically got to a 4v5 teamfight late and pressed Q. the Q alone got me a penta.
Mine was with Vladimir! :D This was around a year and a half ago, and I basically just ulted and spammed E and Q.
Mine was....
NO I AM NOT ASHAMED. (this was before his rework)
Confession time. I have been playing since season 1, and have never had a pentakill.
In my defense, I play almost exclusively top lane tanks and bruisers, as well as a fair bit of support and tank junglers, so not exactly pentakill farming champions...

It was really weird. I fountain dived.