The Red Devils lead second-placed Manchester City by 15 points and are within touching distance of securing the title.
"I think you have to say the United team is as good as it has ever been because nobody normally wins the title so easily and by so big a margin, or has this much of a margin at this stage of the season," Allardyce told reporters.
"It is pretty scary what total they might actually finish with. They have been outstanding and their squad rotation has been Sir Alex's masterpiece, as usual - about playing the right team at the right time."
West Ham take on United in the Premier League on Wednesday.