The ONLY reason why there is even a contest, a discussion between Pep's Barcelona and Zidane's Madrid is because of Zidane's Madrid 3 UCLs, Guardiola's Barcelona 2 UCLs anyway, so it is not like there is a significant gap between these two teams in this matter. Take in account the dominance and football played and the significant. Real rarely played or felt like the best team in the world, their strenght was far more in being able to win even when outplayed and playing badly due to luck and, efficience and endurance than any footballing brilliance, like their 2018 CL run specially showed. 2017 was their only title that had slight hints of brilliance, and even these hints are far inferior to Pep's Barcelona heights. Barcelona needed far less luck than Zidane's Real in their UCLs runs, even when Barcelona lost they still felt great, sometimes the best team in the world, like the Inter or Chelsea ties. Inter was able to win based on many counter-attacks, Barcelona did not play to their full potential and were without Iniesta, they lost 3-1. If Inter had won with only 2-1, Barcelona would have gone through, because a 2 goal advantage in the agregate allowed Mourinho to park an impenetrable bus in the Camp Nou, a game when Barcelona also did not have Iniesta, that he could not have done, at least to the same degree of totally abdicating from any attacking ambition, if he had not such a large advantage from the first game. Even then, Barcelona did not go through to the final with very fine margins. They would extremely likely have won 3 UCLs in a row and any argument for Zidane's Real superiority would cease totally. Or if Real had lost in the penalties to Atletico in 2016 or the final against Liverpool in 2018, or if the second leg between Juventus and Real had gone to extra time. You can say that Barcelona had luck a few times, the 2009 Stamford Bridge game against Chelsea is a scandal, but incidents such as these were far fewer and between than in Zidane's Real because Barcelona virtually always was superior to their adversaries. Even the Van Persie's red card in 2011, as stupid as it was, masks the truth of that game. Barcelona had a penalty not given and outplayed Arsenal from start to finish. Arsenal was barely able to string 3 or 4 passes in sequence and they did not have a single shot, on goal or off goal, while Barcelona had 20 shots, 12 on goal. And, specially in the first half, Barcelona was not even playing at their abolsute best. For instance, their midfield and final third playing was not being as fluid as it normally was, much of this because Busquets was playing as a central-back alongside Abidal! Mascherano was in the midfield. In the second half, with Van Persie sent off and Busquets moved back to midfield, Barcelona created many fantastic chances and recovered their dinamism. In fact, I think that, considering how Van Persie was barely able to touch the ball and Arsenal could not do a single barely decent attacking move during the whole game, not even a remotely decent fast counter-attack, I am much more inclined to think that Busquets moving back to the midfield was far more responsible for Barcelona recovering their best football in the second half than VP sent off. If Koscielny was sent off, and Koscielny rode his luck in that game, it would make be truly damaging for Arsenal. Arsenal's only chance in the whole match came out of nowhere after VP was sent off anyway. And I am not even taking in consideration the first leg, when Barcelona had a goal wrongly ruled out as offside. Pep's Barcelona did lose 2 matches in a row for the first time since Guardiola was the manager only in 2012! Guardiola's Barcelona won 14 of 18 possible titles in all their 4 seasons!
Besides, if you are willing to look and search well enough, all of the all-time great teams did need a some luck to win what they won. No team is unbeatable. Sacchi's Milan, for example, was with only 10 men and was losing 1-0 to Red Star in the 1988-89 UCL when a fog stopped the match and saved Milan from elimination. They played the game again in the next day and drawed 1-1, Milan advanced on penalties. In the quarter-finals against Werder Bremen, Milan drawed 0-0 and won 1-0 at home only with a dubious penalty. Milan's run to the title in 1990 was extremely complicated and they were rarely brilliant. Sacchi's Milan was also stomped by their local rivals, specially Maradona's Napoli, a few times, and they only won Serie A once. That Milan's brilliance was far more inconsistent than Guardiola's Barcelona, a team that rarely had off days and, when they played at their absolute best, they were unplayable. Even Cruyff's Ajax eliminated Benfica in 1972 with a 1-0 victory at home and a 0-0 away. All the arguments against Pep's Barcelona here when we say that they were unplayable is that they lost a few times too. But no team is truly invincible, but Guardiola's Barcelona felt as such when they played at their best like no other team in the 21st century even remotely approaches. For people here detracting Barcelona's performance against United in 2011 UCL final because United was not a great team, rememeber that Internazionale from 1972 and Uruguay from 1974 were also far from being great teams, they were potentially worse than that United team. Ajax's matche against Internazionale in the 1972 UCL final and Holland's match against Uruguay in the 1974's World Cup are widely considered together the peak of Total Football. But both teams were also quite old and mediocre, full of players in decline. Inter in 1972 had lost to Borussia Monchengladbach with 7-1, a bottle from a Borussia suppoter thrown in an Inter player anulated the match and erased the shameful loss, with Inter eventually reaching the final. Uruguay in 1974 would lost to Sweden 3-0 in the next game and be eliminated in the group stage in the 1974 World Cup. Argentina in 1974, a team that lost 4-0 against Holland in that World Cup, was also disgraceful and payed as terribly as big team can in a World Cup, totally imature, mediocre and without any coordination. Germany was actually the only truly good/great team that Holland faced in the 1974 World Cup, the rest were quite mediocre. Even Brazil was making a sub-par and disappointing World Cup. I am not saying this to detract Holland 1974.