Barcelona a question of taste really, arguably in 2011 they got too technical in possession but 2009 team was fun with Eto'o and Henry with Messi. 2015 team also with Suarez, Neymar with Messi. 2006 team with Ronaldinho was fun too.
Bayern Munich 2020 under Flick was one of the most balanced but also attacking teams I've ever seen. 2013 was great too obviously.
For Man United in 1999/00 and 2006/07 probably were the most attractive seasons. 2000 with not playing the FA Cup missed out on another double and one bad game against Real Madrid in the CL, a rare bad night for Keane, but 97 goals scored in the league and some great attacking football, in their element. 06/07 was dribbling Ronaldo before he became goal focused, and Scholes and Giggs were at the start of their second wind of their career, without being too old.
Obviously loathe to say it but objectively speaking peak-Klopp Liverpool were good to watch around 18/19, heavy metal football. Man City have played great football, best was 17/18 with De Bruyne and Silva as attacking mids was the best. Just hard to care.
Arsenal 03/04 gets the credit, but I think the 07/08 team played some lovely stuff and only finished 4 points behind us in the end. If Gallas had been a better captain.
International football is full of good, exciting teams that never won the big tournament:
2000 Netherlands
2004 Czech Republic
2006 Argentina
2008 Russia
2018 Belgium
Some other clubs that caught my eye:
Porto 2011 when they won the Europa League under Villas-Boas
Athletic Bilbao 2012 under Bielsa
Leeds under Bielsa first PL season
Brighton last season
Leverkusen this season