Henry III Plantagenet. He wasn't the best King of England, but you can't help but feel sorry for the guy. First his second son Edmund died within weeks of his birth, then his eldest son Edward got sick and died that same year. Some people think it was that loss which led to his death in 1246.
How could things have been different if Prince Edward survived? I doubt he could have matched the success of Richard II and Henry IV, but I'm still kind of curious.