For me it would probably be Dream Theater. They have a wide range of styles which would be within the genres of rock, metal, progressive rock, progressive metal, ballads and perhaps others. There's enough moods and whatnot that I think I'd be satisfied. They are great musicians so I never get bored of their material. Plus, I own over 12 albums of theirs spanning 23 years so there's a lot of material there.
What about you?
What about you?