Even though I prefer One, I'd say With Or Without You is the definitive U2 ballad. I'm judging "definitive" in terms of the song people think of when they think of U2. WOWY was the hit that made U2 superstars and is known by just about everybody (even my parents who are totally unhip about music sing along to WOWY). One is close, but it wasn't as big of hit and is thus less well known than WOWY.
Stay shouldn't even be included. Yes, it is one of my favorite U2 songs, but it is pretty unknown outside of U2 fandom. Sometimes is too new--if it's a huge hit it may become a definitive ballad, but time will etll.