for me its a toss up between Walrus and Strawberry Fields, but as where I grew up (UK) Strawberry Fields was never on the double EP Im going to go for the song that blew everybody's socks of, and that was 'Walrus'
Seriously I was like 16 at the time and none had heard anything quite like it, from the outlandish lyrics to the brilliant spoken live radio narrations of Shakespeare (forerunner to ideas on albums like Dark Side Of The Moon)and orchestrations.
Walrus is a truly monumental statement gushing with originality written at the peak of Johns creativity, its extremely powerful and burst from the speakers with supreme confidence with those slurpy cello's and wonderful psychedelic notes from the mellotron, a new fangled keyboard used heavily on MMT (the flutes on Fool On The Hill)
Seriously for me this song changed popular music in a big big way, it quite possibly could have been the kick start for progressive Rock with its neo classical overtones (although so could A Day In The Life) ELO's Jeff Lynn said in an interview that it was the catalyst for him creating ELO..
Fool On The Hill is a pleasant enough song (mums and dads loved it) but didnt kick down any musical doors IMO
whilst Flying and Blue Jay Way are both second rate songs IMO