For me Barcelona under pep Guardiola. It was a great team with players like Deniesta, Xavi, Messi, Busquets, Pique and Puyol. A team with a great dominance in time and almost no weaknesses.
I'm not a fan of any particular club, but if we're talking about prize list, Real Madrid have no competitors. With 13 titles in the UEFA Champions League, he is far ahead of all the others. And by bringing Zizou back as a trainer... there will surely be others.
I think the best team I saw playing was Barcelona from Guardiola's time.
The team played a lot and reached the opponent's goal easily. Everything happened very naturally.
This is an extremely complex subject, throughout history there have been many fantastic teams. In Brazil, Santos in the 60's, Flamengo in the 80's. In germany, Bayer in the 70's. In Holland, Ajax also in the 70's. In Spain, Ronaldinho's Barcelona, Cristiano Ronaldo's Real and Barcelona from Messi, Xavi. and Iniesta. Anyway, throughout history there have been many strong teams and it is almost impossible to say which one was the best.