My boy's and my song is Iris by the Goo Goo Dolls. It's such a great song to cuddle and be cutesy to.
Incubus also has a few really good love songs: I Miss You, Wish You Were Here, Echo, Summer Romance, Stellar.
I'm gonna have to go with xok85xo on I Love You by Barenaked Ladies, as well.
I'll Cover You from Rent is also great...it always makes me cry!