Norweigan Wood (Paul's harmonizing on the "She told me she worked in the morning... part")
No Reply (I saw the LIIIIIGHT)
I'm So Tired (I'd give you everything I've got for a little peace of mind)
Ballad of John and Yoko (when Paul joins in at the end)
Sexy Sadie (Wa Wa Wa)
I Don't Want to Spoil the Party (Though tonight she's made me sad I still love her)
You Can't Do That (Everybody's greeeen)
All You Need is Love (She loves you yeah yeah yeah)
Yes It Is (Gorgeous harmonies all around)
Yes, it's a pretty Paul filled list. In my opinion any song Paul sings on makes it better.
Even if it's only in the background. I've noticed that he sings on nearly all of George's songs, many of Johns and double tracks his voice more on songs more often than the others.