I’m going to be really diplomatic here. On paper, you would have to say Barcelona; they did win six trophies in one season. But in terms of the skill it took in various transfer windows to build the team, you have to say Arsenal. I think they were unlucky not to win more, and perhaps that is a criticism you could levy at Wenger. If only there were a machine that allowed them to play one another.
This is a very straightforward question. The edge must obviously go to Barcelona’s sextuple winners, even if Arsenal’s unbeaten season was indeed very impressive. For starters, Barcelona dominated both in Spain and in Europe, while Arsenal’s Invincibles won the Premier League unbeaten, but lost in both Cups and were unable to win the Champions League in one of the most lackluster seasons in European football (Porto vs Monaco in the final).
As if this were not enough, Barcelona managed to sustain their domination. After winning the sextuple in 2009, Barcelona won La Liga again in 2010 with a European point record and reached the semis of the Champions League. This season, Barcelona are again playing at the highest level, look poised to win La Liga again and are favorites for the Champions League. Meanwhile, Arsenal’s domination only lasted one season and only in England.
Finally, a sextuple is something unique, it had never been done. An unbeaten league season, while a great feat, is far from unique; it has been done, among others, by AC Milan in Italy, Ajax in Holland, Benfica in Portugal and Steaua in Romania.
The greatest thing about Barcelona’s unique achievement is that the core of the team was (and still is) from Barcelona’s youth academy.
The golden generation from La Masia plus some very good outside contributions made this Barcelona side one of the best to grace the football pitch, while Arsenal’s Invincibles are not quite at that level. They’re not even the best team in Premier League (since 1992) history: that honour must go to Manchester United’s treble-winners.