First was an ARAM with Twitch. Had to flash for it. Second was my first time playing Kalista, teammates gave it to me after a fight next to baron. Third and most recent was Tristana. I don't know how it happened, but we were at their inhibitor and I just somehow killed everyone.
Old->
first penta on -> 
Yea, ive been playing for a while xD
2
1
1(urf xD)
4
6
5
4
10
6
13
lost count around 20 xd
4
2
1
1
3
4
dang forgot how many i actually got. i'm sure my 1st one was with Trynda doe. I'm also pretty sure i'm missing out on some. i'm a god at pubstomping low elo and normals XD.
(twice) 
i think
Prob have had more than this, but it is kind of a blur. You remember the troll steals more than the pentas.
My first Pentakill was with Zilean lul.
I've gotten 3 on Annie, one of them was 5 ks against the rest of my team by using Tibbers at JUST the right moment to kill the whole enemy team, how do I remember that one specifically? It was my first ever penta, and hilarious af.
I lost count on Kat at around 20... my favorite was this one time, i came running to the rescue of my teammate who got caught by all 5 of the enemy team, I jumped in and penta killed in less than 2 seconds, they immediately surrendered XD.
I've gotten 2 on jinx, late game op yo.
My first ever time as (new) yorick, I got super fed off their crappy garen top lane, then just button spammed to a penta, have never been able to replicate that since I suck at him so much XD.
I 1v1d the enemy carry, then ulted, got a quadra started laughing, then suddenly penta, I hit the fifth dude while he was recalling completely on accident, couldn't breathe for a bit after that one.
Remember when her ult was bugged to do insane amounts of damage? Yeah so does my pentakill game with her XD
I got a quadra in a teamfight, and was recalling, the enemy jg said "nope" in all chat, then he hit a shroom and I got a ppenta kill, another time i just straight up penta in a teamfight due to an awesome soraka helping me out :b
This one was recent! In champ select I told my team I sucked at zed, and I didn't have any RR's, and our Tahm said he did too and wouldn't trade, I got a kill on yi, then tthier whole team (Ekko, Ryze, Jarvan, and maokai) all took turns diving me under turret and I got the penta, it was so sad since I suck at zed and didn't deserve it at all XD.(this was aram btw)
twice, first time in an aram game, they all dived me, 1v5, and I ulted and out lasted all 5 of them with shield and hextech, it was so great XD, then another time in a normal game I got super fed while jging, and they were Ashe(ultdown), Ezreal, Janna(killed her first surprise attack op), Akali, and khazix, so they had no good cc to shut me down XD
Ever play a game where you are the ONLY even remotely good player? On either team? Yeah I played a game like that as Janna...
And personally, I would trade them all to get the 
penta my friend in gold got a while back, that was freaking hilarious.
I got one on , a champ I rarely play, got a pentakill stolen by a scumbag cait when i was , and another stolen again as
I'm a support main so unless I get incredibly lucky with Zil I'm not getting a pentakill on the rift. I did get my first ARAM pentakill with yesterday though
First penta was with MF in like season 3, entire enemy team took squishies and we had a Jarvan. Second penta was also with MF in the same game. We dove their spawn for that one. First ranked penta was also in season 3 about a day after I bought Yorick... We still lost lmao.
I've gotten like 8 pentas in URF with Garen since his e is broken in that mode. Take ghost and it's got 0 counterplay. Level 2 double kill in 2 seconds. Think I've gotten a penta or two with the usual penta champions, Kat and Yi. I definitely got one with Kha a couple weeks ago... Might've gotten one with Ryze after his first rework when he only had one combo--qeqwqeqwqeqwqeqwqeqwqeqwqeqwqeqwqeqw
I don't keep track of ARAM pentas since they don't really count.. Guess URF pentas don't count either tho
3 x before the rework
1 x
2 x
1 x 
which was the, funniest penta i have ever gotten in my lyfe
ooo after reading your post I remembered I got one with Nunu in the baron pit. We got aced and I was the first to respawn. Flashed into an Ivern bush and ulted their team for a penta lol
Jinx, but it was on Hexakill and the enemy Garen just barely got away...
Xerath, in a ranked game that I didn't die in as I recall.
Nasus, where the last man standing was the enemy Maokai and it came down to the wire on who would kill who first.
