Moltres has always been my favorite, but recently I've come to really like Articuno. Just a beautiful Pokemon all around, who cleans up in battle as well. Doesn't get much better than that :D
My favourite is Moltres. First and foremost, I like the name Moltres. I'm no etymologist but I'm pretty sure the name derives from "molton" and "très" (French) which makes "very hot". Moltres is a very hot bird and I like hot birds.