I predict the replies on this thread will be quite generational.

I have to give you six.

The Beatles captured me with their composition.
Bob Dylan is on the list for his lyrics.
Jimi Hendrix is there for his use of electronics and innovation.
Chicago is there for their horn arranging.
Pink Floyd is there for their production.
Styx is there for their ensemble vocals.

Bonus pick:
Todd Rundgren is there for his "all of the above".