God, I can't even choose, his voice is so amazing, any song he sings on makes it better. He definitely had the best voice of the 4 of them. So versatile. Here's a couple though:
I've Just Seen A Face
Here, There, and Everywhere
If I Fell
I Don't Want To Spoil The Party (I know it's not his song, but the "Though tonight she's made me sad I still love her..." part he sings on is just amazing.
She's Leaving Home
Long Tall Sally
Blackbird
I Will