Aaaah a tough question! I'm lately in a rather odd place in life and so am pretty much only listening to Maiden (they are the only band that I have no emotional ties to any other person or event). There are bands where I absolutely love one album, and if they had made only that one I would class them as a favourite band, but I find the rest of the material lackluster etc.
Top 3 are probably, in no particular order: Iron Maiden, Judas Priest, GnR.
Other ones that are high on my list however: Aerosmith, Alice Cooper, Motley Crue, Megadeth, W.A.S.P, ZZ Top, Halford, Bruce Dickinson, Sixx.A.M (theres only about 1 track theyve done which I dont like!!! But I just can't place them above the 3 above for whatever reason), Shinedown, Queen, Dio, AC/DC (Love most their stuff, but not listened to in years for some reason?)
Never been able to get into Metallica at all... probably not helped that the first thing I heard was Saint Anger
but theres only about 5 songs I can even remember what they go like, and 1 of those was written by Dave Mustaine so...
Not been to any gigs in a long time, but I've seen the vast majority of the above live.