The greatest game played by the Colts was the 1958 World Championship. No arguments from me.
The second greatest, IMHO, was the 1977 season ending cliff-hanger against the New England Patriots. Fresh off an embarrassing (and some say tainted) loss to the Lions via blocking David Lee's punt and the ensuing controversy about eliminating the Patriots by losing.... the Colts were in a must win situation. Then proceeded to lay an egg in the first half, and remarkedly similar to SB 47, dug themselves into an 18 point hole by getting torched on the opening kickoff of the second half.
Then Bert Jones led what was one of the most remarkable comebacks in Colts history -- and the only one which clinched a division title -- starting with a bomb to Raymond Chester in the face of an all-out Patriots blitz (see 0:09 below) -- and ending with the controversial "in-the grasp" call.
That was the last regular season Colts game I ever saw, since I moved to New Orleans the next season. Along with the Miami "fog game", the Bert Jones to Roger Carr & Glenn Doughty & Ray Chester offense plus the Sack Pack brought us thrills which were not felt again until at least 2000.
Anyone have another game which we should consider as the 2nd greatest?