I thought I'd get the ball rolling and vote for Ask Me Why.
It might be a strange choice, I know most people will go for others. I just absolutely LOVE Ask Me Why, more than so many other Beatles songs. Always have, always will.
I think it's special for me because of the way they've crafted it so perfectly. The harmonies are just fantastic, especially the way they swagger the words with the whole "I love you-woo-woo" thing... so swoon-inducing!
And as I adore John, I enjoy it extra because John takes the lead vocal inbetween the harmonised sections. I don't need to go into detail about how awesome his gravelly tones are.
It's just a gorgeous love song that plays on so many musical devices and feelings that make me completely fall in love with it, and the band even more than I already am.