Definitely the 2011 Heat. After holding a parade and promising multiple championships, they absolutely ran through the Eastern Conference playoffs. After they lost Game 1 to the Bulls in the ECF, they murdered them the next 4 games. None of their first 3 series were even remotely competitive. Heat championship rings were probably already halfway done being crafted before they even played in the 2011 NBA Finals.
Game 1 of the Finals vs. Mavs they dominated.
Game 2 they had a 15 point lead with 7 minutes left.

Game 3 Bosh hit a game-winner to take a 2-1 lead.
And then LeBron got outplayed by Jason Terry for 3 straight games.
Injuries happen all the time in sports, and when they occur, it sucks. It was definitely a disappointing season for the Lakers, but are we going to sit here and say the 2008 Pats were a massive disappointment after going 16-0 the previous year? No, because Brady got hurt... Similar thing here, except it happened with a variety of guys and some of them simultaneously. There were no delusions about the Lakers' destiny less than halfway through the 12-13 season. And once Kobe went down, it was a wrap. If we see them flaming out in December/January, and then it actually happens in May, is that really all that disappointing?
The 2011 Heat choked something fierce. Can't remember a time a team had so much preseason hype, actually came through with it throughout the regular season, stepped up in the playoffs, only to fall short on the biggest stage. Being the first team in NBA Finals history to lose it all after taking game 3 to go up 2-1 (in 3 straight games no less...), THAT is what you call disappointment at it's finest.
And then Dirk and Cuban styled on those fags by going to LIV and celebrating
Edit: The fact that everybody remembers the 2011 NBA Finals for The Heat/LeBron choking instead of the Mavericks winning tells you everything you need to know about how disappointing that team was.