Well - I am in a totally monogamous relationship and plan to stay that way and I don't usually fantasize about guys outside my honey.
BUT...if I was going to - it would be a long list.
Richard Speight Jr (the Pepsi Max commercial guy and Gabriel from Supernatural)
Patrick Stewart (Captain Picard from Star Trek: TNG)
Jim Beaver (Bobby on Supernatural)
His name escapes me - but he plays Agent Booth on Bones and was Angel
oh - and the guy who played Balthazar on Supernatural - wow - I loved his attitude and when they killed him off I got so mad.
Ooops - forgot - Mark Shepard (spelling) who has played Crowley on Supernatural and been in Firefly (Badger), Battlestar Galactica, Dr. Who, Leverage and I forget what all else.