My only penta was with Vayne, but that was in ARAM so nobody cares. Aside from that, the closest I ever came was as Olaf. I got a quadra and was still healthy. I was on the last guy, but my allied Garen hadn't ulted yet, so yeah. Sadlaf.
Jayce, it was an unofficial penta, the other team had a super fed karthus who was extremely ballsy and decided to go head-on first into baron while we were doing it and their team was 3 seconds behind of him so I picked them off one by one
edit: i've been playing since before or during the talon patch so about 8/24/11 so ive gone almost 2 years without a penta