Addition to the 83 Redskins, the two teams that ACTUALLY ADVANCED to the Super Bowl and lost with the strongest case(to me) are the 2001 St. Louis Rams and 1968 Baltimore Colts.

2001 St. Louis Rams
-14-2
-No. 1 offense in the NFL, no. 3 defense
-Per-game point differential: 31-17
-No. 1 passing offense, no. 1 YPA, no. 1 rushing TDs and YPA
-No. 2 in allowing 1st downs, no. 3 in defense against rush
-Kurt Warner, Isaac Bruce, Torry Holt, Marshall Faulk, Ricky Proehl, Az Hakim, London Fletcher, Aeneas Williams, Jeff Wilkins and Mike Martz-the pinnacle of the Greatest Show on Turf era
-Almost overcame the biggest deficit in Super Bowl history(at the time) to defeat the Pats

1968 Baltimore Colts
-13-1
-Defeated the Vikings 24-14 and Browns 34-0 in NFL playoffs
-Per-game point differential: 29-10(absurd)
-No. 4 in total offense, no. 2 in total defense
-Permitted only 144 points in 1968-league record at the time
-No. 4 defense against pass, no. 3 against rush, no. 1 against passing TDs and YPA
-Backup QB Earl Morrall won MVP(Unitas was injured all season)
-Also featured Tom Matte, John Mackey, Jimmy Orr, Bill Curry, Mike Curtis, Bubba Smith, Rick Volk and Fred Miller(LSU boy)
-Committed 5 TOs and missed 2 FGs in a very winnable Super Bowl

What is often obfuscated about the 83 Redskins is that team was no. 1 in the NFL at acquiring INTS, no. 1 against rush, no. 1 against rushing TDs, no. 3 in total rushing, and no. 1 in rushing TDs, truly remarkable.

Best team not to reach the Super Bowl ie truly underachieved, probably the 87 49ers

1987 San Francisco 49ers
-13-2
-Per-game point differential: 31-17
-No. 1 total offense, no. 1 total defense
-Led NFL in passing TDs, passing YPA, and rushing
-No. 1 defense against pass, passing TDs, and 1st downs
-Bill Walsh, Joe Montana, Steve Young, Jerry Rice(the 22 TD year), John Taylor, Brent Jones, Dwight Clark, Roger Craig, Tom Rathman, Charles Haley, Ronnie Lott, Michael Carter, and Keena Turner

This post was edited on 5/25/17 at 4:25 pm