I got my first ranked penta a few weeks ago too. Was with vayne. Before that i had only gotten Pentas in normals with akalis. I think my count is
MF: 1
Vayne: 1
Kog maw: 1 (My first ever, back in 2010)
Akali: 4
Ahri: Quadra kill and then shot the Q the wrong way and died i was so embaressed.