In terms of a team loss, I can't think of any loss more devastating than 2010 Finals, Game 7.
Losing in 1987 to the Lakers was tough -- but (to me) less at the time than when viewed historically. We didn't know at that moment that we wouldn't win again for more than 20 years and that Bird would be forever behind Magic in championships as a result of that loss.
Game 7 this past June was just plain agonizing. The loss of Perk in Game 6 isn't an excuse --it's part of the game -- but, despite that injury, we came so close. Adding insult to injury, the Lakers creep within 1 championship of the all-time lead. Worst loss ever as far as I am concerned.