His HOT accent/voice
I can't get over it. I think he might have the sexiest voice I've ever heard.
I recently got a new DVR that will record anything with any actor whose name I speak into the remote. I added him to my list. He was "in" an episode of Family Guy, so it got recorded. I don't care for Family Guy, and don't normally watch it, but decided to do so before deleting it. Even in something as silly and dumb as FG, just the sound of Liam's voice made my nether-regions practically vibrate! I'm not sure there is another whose accent affects me quite as physically as his.