Right now, it's Symphony X, particularly pre-Paradise Lost Symphony X. Stuff like this is just sublime:
[video=youtube;lBqfbCFcPTo]http://www.youtube.com/watch?v=lBqfbCFcPTo[/video]
I love lots of other bands, though. I have a soft spot for Iron Maiden, as they got me into metal. Seventh Son of a Seventh Son is my favorite of their albums.
Dream Theater is another favorite. Again, I prefer their earlier (pre Six Degrees of Inner Turbulence) albums, but their latest, A Dramatic Turn of Events, might be their best since their concept album, Metropolis pt. 2: Scenes from a Memory.
Finally, there's Savatage, whose output is so diverse that it's hard to believe they're the same band by the late 90s that they were in the early 80s. Highlights are The Dungeons are Calling, Hall of the Mountain King, Streets: A Rock Opera, Edge of Thorns, Dead Winter Dead, and The Wake of Magellan. That's what, half their output? Yeah. Fantastic band.
Other great bands are Theocracy, Sonata Arctica, Blind Guardian, Galneryus, Kamelot, Candlemass, Cathedral, Trouble, Nightwish, Metallica (pre Black Album), Opeth, Ayreon, and Avantasia. I'm sure several are missing from my lsit.