Iron Maiden since 1991....
Some other favorites are:
Amorphis (older), Dark Tranguillity(older), Dimmu Borgir, CoF(older), Emperor, Dissection, May Result, Opeth, Opera IX, Nile, Nevermore, Dark Funeral, Satyricon, Agathodaimon, Ancient Ceremony, Thetatre of Tragedy(older), Pink Floyd, Black Sabbath... There are more but I can't remember all of them now. I don't listen to Maiden that often anymore, but they have the most sentimental value, so I put them on top. Musically, I prefer more extreme stuff.