That Red Star team as a club team is certainly the best one I've seen in a real-time environment, or in my lifetime so to speak. In terms of club teams there is also, in my opinion, a case for maybe the Milan team of the late 1980s early 1990s (the last team to retain the European Cup / Champions League) and possibly the Bayern and Ajax teams that won three in a row each in the early 1970s. There was a River Plate team in the early 1940s called "the Machine" (La Miquina), they won a lot of Argentine Leagues (at a time when it could be considered one of the hardest leagues in the world) - they didnt win any continental trophies because the Libertadores wasnt foudned for another 20 years (1960). There was a South American Champions Cup (called the Campeonato Sudamericano de Campeones) arranged as a one off in 1948, but the River team was too old then, about 5 years past its prime, they were playing with 38-42 year olds and they came second. That River team of say 1938-1943 would be amongst the strongest club ever, but obviously it's hard to judge it on the international stage.
Every team does have an "ugly" side, and some of those teams played football that wasnt perhaps that astectically good. But I dont consider this Barcelona team to be "the greatest ever", simply because there now appears to be a distinct way to beat them, an achilles heel, a way of unbinding the Gordian Knot so to speak. Last season we saw Milan pretty much nullify them as an attacking threat for much of the two legs, Chelsea went further and schackled them with additionally being able to score goals themselves on the counter attack, and playing a huge part of the second leg with only 10 men. It's not really anthing vastly new either, the blueprint was set up by Berdiwyev's Rubin back in 2009. They dont really have a plan B and so if you can pack the midfield, if you play a massivly defensive formation, you can stifle them and they will struggle to play a more direct style of football.
If you look at the other teams mentioned, teams like Red Star and River, and the Milan team. These teams could really adapt their type of play. The best example is Red Star, playing scintillating football against Bayern in the semis, before grinding out the 0-0 and winning on penalties vs Marseille. It mightnt have been great to watch but Red Star knew that this was probably their only chance of winning the European Cup, by Summer 1991 the writing was on the wall for the Yugoslavian Federation, everyone knew the country, the leagues and all the teams would split and they would never again be able to challenge at the top of European football. The pressure on one match became too much and instead of playing free flowing football under the arches of the San Nicola in Bari, Petrovic told them to stagnate the match and make sure they didnt lose. Similarly the Milan team of the late 1980s and early 1990s were famous for having a tight defence, they were reknowned for "grinding out results". Yet this wasnt some robotic lumbering team. There were quick attacking skillfull players like Donadoni, Costacurta, Tassotti, players who could switch from systematic defence to lightening quick attack at a moments notice. Also at the heart of this team you had the three Dutch players (Gullit, Rijkaard and Van Basten) again players who could adhere to defensive systems but also provide wonderfull attacking outlets. Especially Van Basten, probably the finest Dutch striker in the last 30 or so years and one that was cruelly blighted by injury. With this sort of squad Milan could play both the defensively organised game they were noted for (beating teams like Mechelen*, Bayern and Bayern by single goals) and also switch to the dynamic attacking play when needed, putting 5 past Real in the second leg in 1989 having drawn 1-1 in Madrid. I dont see an ability to switch styles in that way in this Barcelona team.
* Who were then a very good side