Mine was with Kog'Maw. It was about... midgame, I was snowballing pretty heavily, I think I was something like 12:3. So, the entire team enemy team comes to gank our Tryndamere jungler, who is also snowballing at around 8:3, at wraiths. Well, he winds up getting in some AMAZING crits the likes of which I've never seen before or since, and deals massive damage to their entire team despite their soft CC slows spreading out his damage so much. So, when I waddle up, to clean up, I E and pick off 4 champs in the space of sub-5 seconds, then severely damage the 5th one before I die, only to kill the 5th and secure the penta with Kog's passive xD I fondly remember going "OMG OMG DID U JUST SEE WAT I DID LOOOL" to my friend who was playing next to me at the time. He was so envious of me afterwards because he wanted to be the first to get a penta in PVP!
My first penta was with Cass. I remember getting the 4 kills at once right outside of their bot inhib tower. The last person to kill was warwick and he charged down bot lane (back then the timers for pentas was longer). I chased after him and another team mate came (but he almost ksed). Warwick ran into a bush and I q'ed it and got the penta. According to him, 2 more seconds and his ult would've been up lol. Since then I've gotten numerous, but that was my first.
I am a lot more attracted to the support role so I am not that much in search of kills but I have to admit that the first time I was able to do it I felt great about it and it's actually one of the events that made me that much more interested and made me want to keep going with my new found game. I consider it just luck though since my teammates did all the work and I was just in a beginner rank so I assume most of us if not all were just noobs.
I've only been able to do quadra kills up till now, despite playing the game for a long time. Besides of that I did make several pentakills when playing cooperatively with other human players against bots, but I guess that doesn't really count. It would be cool to make a pentakill with a difficult to play character. Perhaps Master Yi would be just too easy and be considered almost "cheating".
Nonexistent. Not a single one in 3+ years of gameplay. Maybe I could get one if I made a smurf. Either the ADC or the support steals it. I think around 4/9 of my quadra kills could have been pentas if not for that. I swear I'm not salty. At all. Not a single bit. Don't mind me.