I couldn't pick just one.  Would depend on the era.  There's a lot of bands I would also consider equal, as in equally good.  I'd groups the band like REO, Styx, The Cars, ZZ top, etc in that group.  Then you'd have some a bit better, Van Halen, Lynyrd Skynyrd, Boston, Journey, Foreigner, U2, Pearl Jam, Soundgarden, Metallica etc.  Then the best IMO would be bands like Pink Floyd, Rush, Guns N Roses, Beatles (not my favorite but a lot of hits),  Led Zeppelin, Rolling Stone etc.  Now I'm not saying the best one's are my favorite, but would consider them the best by their hits and techniques and what they added to rock and roll.