Ooh, this is too hard... I think all these bands are spectacular, I really do. My brother introduced me to metal with Iron Maiden. Specifically the Brave New World album. Bruce is a great front-man and they have great consistency when it comes to songs as very few in my opinion are below average.
Black Sabbath are pretty good, but I have to admit that I've never really listened to them that much.
When it comes to Raw metal power nothing beats Motörhead.
Still, since I have to pick, it has to be Judas Priest. Rob's vocals, the riffs, lyrics... I never get tired of Priest (Admittedly, Breaking the Law is ridiculously overplayed/rated).
Also, the painkiller album is godly.