United, champions for the last three seasons, are involved in the most open and unpredictable title race for years, typified by their 1-1 draw at Birmingham City on Saturday when they failed to get the win that would have taken them top.
Arsenal, who could also have inched closer to Chelsea after the leaders had their trip to Hull City postponed, slipped up in the snow of the Emirates where they were held to a 2-2 draw by Everton.
"There is an improvement in the league. You have to assess it that way," said Ferguson. "The general standard has improved."
Wenger said: "Something has deeply changed this season. There are now more teams aspiring to be in the top four - that has completely changed the structure."
"There are five or six clubs that can win the Premier League," Mancini told reporters.
"The league is more competitive because there are many top players in England and, of course, City are a top club."
"It's a difficult place to go to at the moment, it's not a bad point (for us)," he said. "It's a tough league, it's not going to be easy.
"Teams like Birmingham have really improved since coming into the division."
City, the world's richest club backed by the billions of owner Sheikh Mansour bin Zayed Al Nahyan, added former Arsenal title-winner Patrick Vieira to their squad on Friday and he also believes the title race is wide open.
"Manchester United do not dominate the league like they used to," said the midfielder.
"Look at the league - we have Chelsea, Liverpool, Arsenal and now Manchester City, Tottenham and Aston Villa.
"They have made it really difficult for United to dominate and I strongly believe we are not out of the title race here either.
