I'll put it this way:
Nirvana helped define a decade, as well as a sub-genre of a genre of music...
The Beatles defined and *transcended* a decade, defined a whole genre of music, and branched out to define other elements of culture today, everything from advertising to the way we view stars to film ("A Hard Days Night" actually got a release from the Criterion Collection...which admittedly would be more impressive if they also hadn't released "Armageddon" as well, but hey, no doubting that, comedic though they are, both "A Hard Days Night" and "Yellow Submarine" are two films that helped define 1960s pop cinema...especially YS, actually, that's STILL some amazingly-impressive and original animation!)
So, yeah.
Beatles.
Beatles all the way...until the next vote, when I'll of course vote Beethoven for champ. :p