Barca 2009 was not as great as people say. I actually think it was the worse version of Barca under Pep, and worse than the other treble winning team (the Luis Enrique one). It is just that the team became so good after that season, but did not manage to win the Spanish Cup when they won the other too that is gets mentioned in these discussions. Barca 2011 was significantly better in pretty much everything.
In fact, Barca 2009 would not have made the final of it wasn’t for the biggest crooked referee performance in the history of UCL. And while they defeated United fair and square, I think we were not as good as the previous 2 seasons. We won the league only with 80 points, dropping a significant big lead, got humiliated by Liverpool in Old Trafford, and were barely able to go past Porto in the quarters. We were not as good as the previous season IMO.
Barca 2011 is the best I have seen, but it didn’t win the Spanish Cup so out of the competition. So I would go with Bayern 2013 as the best treble winning team, closely followed by City 2023. The others a bit below them, but honestly all were so good that any team could have defeated the others.*
* Blasphemy to say in this forum but United 99 was the worst in the sense that the football had evolved so much (keeping the ball forever, pressing) that they would have had an extremely tough time against ball possession teams (all except Inter 2010), and in a league between these teams where every team plays the others twice, I think United would have ranked last. But then, I think that United 2008 was quite better than United 99.