Gotta say Dream Theater takes the cake for me. I see that Rush has taken a beating, so I'll just delicately say that I do enjoy DT's musical proficiency a lot, but that's not the sole reason why they're number one for me. Their lyrics and songwriting speak to me in ways that other bands don't. It's just a matter of taste, really. If I'm feeling down, my mp3 player somehow shuffles to a song by them that fits my mood and usually winds up helping me along.
But, in other tastes, I'm pretty diverse. I am very fond of traditional Irish music (Chieftains, for example), but can just as easily enjoy the growling of Opeth, and then hop to some good ol' Bob Dylan.