Beckham captained the MLS All-Stars against his former club this week.
"They are the biggest team in the world and they are the best," he said.
"There is no other team like them. I have been lucky enough to play with AC Milan and Real Madrid but there is no other club with that attitude. Any new player that comes into Manchester United feels it straight away.
"The manager expects you to work hard and win games. It is instilled into you at a very young age at Manchester United and it carries on all the way through.
"There is a never-say-die attitude. It is what Manchester United have."