Thor wasn't able to take on the Celestials simply because he had a strong arm. It was his very nature as a God that allowed him to operate within their sphere. The Hulk and Namor are not cosmic in scope or power and therefore would likely not fare well in single combat with a being of cosmic proportions.

However, in regards to sheer strength and fighting prowess, they have proven themselves on a par with the Thunder God.

I find an over-reliance upon playing the "implications game" to be a serious detriment to good comics writing. If Robin turns 22 over here, but you still need Jimmy Olsen to be 18, then screw it. Keep Jimmy 18. Let the heavens and earth fall. Let the righteous fury of fandom assembled align itself against you. Operating within a shared universe should not carry with it a blanket obligation to slavishly cater to whims granted by one editoral team without your approval. Hell, should the book fall into your hands, make Robin 18 again yourself.

Yes, fans will snark, but if the story is a better story with a young character and you don't want Batman to be pushing 50, then accept that you're writing fiction, and write the story the way it works best. Some editors believe that only a strict adherence to Continuithink is "professional." They're wrong, but what the hell. Write it their way until they self-destruct over the size of Cap's shield and then write it well afterwards.

Just because Thor beat Namor in FloppoGuppies Annual #2 in 1962 does not mean that every writer thereafter is beholden to that outcome. In fact, you do yourself and the readers a disservice to remove the concept of imagination from the equation and simply check the box that says, "Yes, I read and shall abide by FloppoGuppies Annual #2 and all it's addendant implications, now and forevermore..."

Readers pick up the book to see a contest. Let it be one.

I tend to think of it in terms of sports teams.  On any given Sunday almost anyone is defeatable.  How many times have the Packers smacked the Lions in the mouth?  And how many times has the reverse happened?  Not perhaps the best example, but I believe the concept is valid.  As long as we're not talkin' Daredevil v. Galactus here, I'm willing to accept that there's enough power amongst our heroes that the difference between success and failure is miniscule.  Whichever way the coin flips that day decides it. 

Or, whomever's book the fight takes place in wins it.