Think the Bayern treble side deserves a mention here somewhere. They came back with a vengeance after finishing everything runners up the year prior and were utterly ruthless. Their finest moment was systematically dismantling a close to peak Barca side that easily won the league that year and were one of the favorites to win CL.
That Bayern team had incredible tactical versatility. I think that they had the second best possession and pass completion stats in Europe after Barcelona that season. They had the ability to press high up the pitch, or sit deeper and spring rapid counter-backs. It was like they had an ideal combination of skill, speed and strength.
Of the 5 trebles that I've seen, Bayern's in 2013 was definitely the most dominant.
Utd's in 1998/1999 was of course the most exciting and dramatic across all 3 competitions, with the thrilling title race including coming from a goal down to beat Tottenham on the final day, FA Cup drama against Liverpool and Arsenal, CL drama against Juventus and Bayern etc, but that's not to downplay the tremendous quality of course.
Barca in 2008/2009 had the controversy at Stamford Bridge in their CL semi-final, and 'wobbled' slightly when they had opportunity to secure the La Liga title themselves at the Camp Nou, failing to hold on to a 3-1 lead at home to Villarreal. The champagne was put on ice although a week later they were champions after Real Madrid lost away to Valencia.
In 2009/2010 Inter from memory had a 13 point lead over Roma in Serie A just past the halfway stage, before Roma pegged them back and moved 1 point ahead with 4 games left, but lost at home Sampdoria, allowing Inter to regain the lead and seal the title on the final day of the season.
Barca in 2014/2015 looked to be in crisis around the turn of the year, especially after that defeat to Moyes's Real Sociedad, with Enrique and Messi reportedly feuding, Enrique constantly chopping and changing tactics and close to getting sacked, before he settled on a fixed starting line-up with Messi on the right and Suarez in the centre, and that front 3 became unplayable during the 2nd half of the season.
In 2012/2013 Bayern were just a ruthless, unstoppable, all conquering juggernaut. Yes that they had that scare against Arsenal in their CL last 16 tie, and they were clearly complacent going into the 2nd leg, but I did think they were the clearly the better team over the 2 legs and so weren't lucky to go through.