DARIUS!! Just let your team do the work then dunk dunk dunk dunk dunk PENTA KILL

The answer is always Karthus. Especially in this case. :P

Actually my only pentakill came from ad nidalee a few weeks ago, But i have had a ton of quadras with Mord/nidalee/amumu

I've achieved "official" penta kills with 3 champions:
1. Karthus
2. Morgana
3. Caitlyn

Karthus penta was picked up without using my ult, Morgana was a level 6 penta kill when the enemy team dove into our tower, Caitlyn wrecked in a team fight.

I got a pentakill with Caitlyn, Zyra, and Yi. All of which were pretty simple due to great positioning and fighters to keep me safe.

Katarina, Darius, Akali, etc.

Most champs that kill secure are more likely to get penta kills.

Only one I've ever had in ranked was Xin

My only Penta has been with Cass, seen multiple Yi and Akalis. and a few Kha'zix

The closest I've even gotten a Pentakill with a champ are:
Mordekaiser
Heimerdinger
Graves
Twitch.
I think Graves, Mordekaiser, and Darius can be easy pentakill.

I have gotten a pentakill with garen and darius