The treble!
During United's 1999 season they proved that they were the greatest team in Europe, beating the likes of Beyern Munich and Juventus on their way.
Arsenal only won one trophy that season, having flunked in the Champions' League and FA Cup.
It's easier to win a league match against the likes of Man Utd and Chelsea than to win a Champions' League match against them. When it came to the big matches Arsenal messed up, whilst United excelled.
The unbeaten season was a brilliant achievement, but it's the number of trophies that's most important every time!