Probably Richmond 1927-1930.
If Collingwood 4-in-a-row is the best team in history (there's another debate), then that Richmond side (who were beaten in 3 of those GFs, 1927-1929, and got knocked out in the SF by Collingwood in 1930) is probably the best side to miss out. They also lost the GF in '31 (to Geelong), but won in '32 and '34.
It depnds on when you define a team's era stops.
If Collingwood 4-in-a-row is the best team in history (there's another debate), then that Richmond side (who were beaten in 3 of those GFs, 1927-1929, and got knocked out in the SF by Collingwood in 1930) is probably the best side to miss out. They also lost the GF in '31 (to Geelong), but won in '32 and '34.
It depnds on when you define a team's era stops.