I'm looking at this because it was discussed at Baseball Think Factory. That discussion was inspired by a new website that is trying to get Munson in called VoteThurmanIn.com.
Munson is 14th in career WAR among catchers. Here is the top 22 from Baseball Reference (I went to 22 so I could include Campanella). It also shows their best three consecutive seasons (which I just eyeballed to find) and the number of times they finished in the top 5 and 10. These guys played at least 50% of their games at catcher.
Now Munson is only 240th among position players in career WAR. That probably is not enough for the Hall. But catchers don't have the longevity of other players, so being in the top 15 is impressive. If he had not died at age 32 in 1979, he might have made the top 10. He was on a pace to get about 3.6 WAR in 1979. He also had over 3 the previous year.
His best 3 straight years is not near the elite like Bench, Carter, Piazza or Mauer (does he have the record for best 3 straight by a catcher?-although some games at DH may have helped). But Munson's best 3 straight years beat Lombardi, Bresnahan and Hartnett. He also has more top 10 finishes than Lombardi and Hartnett.
Tenace is ahead of him in career WAR but only about 58% of Tenace's innings were at catcher (Munson is about 97%).
Munson also is 144th in MVP shares (at 1.50). That is good considering that catchers don't do that great in the voting. See my post called MVP Awards And Award Shares By Position. So his contemporary observers liked him. He also won three Gold Gloves.
Fangraphs has him at 21st in career WAR among catchers. But some of the guys ahead of him played less than half their games at catcher: King Kelly, Joe Torre, Brian Downing, Buck Ewing. Tenace and Bresnahan are both ahead of him, too. Bresnahan played about 70% of his games at catcher (this all points out something problematic with the position regarding the Hall of Fame-many guys who played there got moved to other positions). And again, don't forget that Munson died young. He could have moved up in the rankings.
Just what I have done so far is probably not enough. But I think we should be looking at him as a serious candidate.