"Taxman" (The Beatles)

Although "Taxman" from Revolver (1966) wasn't George Harrison's first foray into writing songs, I think it is the song where he started to step out of Lennon and McCartney's shadow.  There's a real sense of confidence and bile in it: Harrison was pissed off about being in an income bracket in the UK where the majority of his earnings were going straight to taxes, and he let fly with this terse, scorching editorial.
You can't completely count out the other Beatles' contributions, though—particularly McCartney.  Macca does double duty on the track, cranking out the hellacious, Stax-indebted bassline (the man sounds like he has four calloused thumbs on the section from 0:55 to 1:06), as well as the raunchy, Eastern-tinged lead guitar solos at 1:12 and 2:25.
For me, it's the best Beatles album opener, next to "Come Together."