Eleanor and Park.
It's Eleanor and Park hands down.
You can find my review of the book here (scroll wayyyyy down.)
But let me tell you why I think that this is the best love story ever - because real love stories happen between imperfect people. Eleanor is big, badly dressed and can't afford a toothbrush. Park is half-Korean, obsessed with comic books and isn't havin' any of the whole popularity nonsense in their high school. They are both kind of messy.
Also, the writing is incredible. How about sentences like, “Holding Eleanor’s hand was like holding a butterfly. Or a heartbeat”?