I'm sure this has been brought up before but I don't recall seeing it. I have stated in the past that I don't believe you can compare eras so I have broke it down to 3 categories; Modern Era 18 game season, Modern Era 16 game season and finally BC (Before CFL).
Here are my picks:
Modern Era 18 game season: 1997 Toronto Argonauts - 15-3, Grey Cup Champs, 660 (36.7/game) points for-327 (18.2/game) points against
Modern Era 18 game season: 1981 Edmonton Eskimos - 14-1-1, Grey Cup Champs, 576 (36/game) points for- 277 (17/game) points against
Before CFL(1958): 1948 Calgary Stampeders - Only undefeated team in history
If I had to pick one team, my personal choice would be the 1981 Eskimos but I do admit to a slight bias.