Niki Lauda has tipped Mercedes to flirt with the top positions in 2012.
After February's winter group tests, rivals and pundits are now assessing the likely pecking-order less than two weeks before the season opener in Australia.
"My feeling is that McLaren and Red Bull are almost equal," triple world champion Lauda told Austrian television ORF's Sport am Sonntag programme.
"The big positive surprise is Mercedes. They have chosen a different way," said the 63-year-old.
"The nose is relatively small, making the car look quite different to the others, because they have the (front wing innovation) F-duct.
"For me, it's the most innovative car, and it also looks to me as through the (F-duct) mechanism works very well."
Lauda said a strong Mercedes will be the first real test for Michael Schumacher since the great German returned to F1 in 2010.
"It's a very important year for him," he said, "because for the first time he and us will really know the answer to how he stands against Rosberg.
"Michael needs to prove to us all how good he really is -- which of course is difficult, since logically you do not get faster with age."
"They're behind, no doubt about it," said the Austrian. "The car is pretty unstable, slipping at the front as well -- I think they could be two or three tenths behind the top three.
"At the beginning of this year, that's quite a lot," explained Lauda.
"In Alonso they really have a top man, but if the car is too far behind then he can't do anything about it, especially as he has a not-so-strong teammate in Massa who is not going to push him."