they went 22-3 in 2011 (statistically the best season we've ever had),
Geelong Premiership Teams: Ranked by match/win percentage
Interestingly both the 1952 and 2011 won matches against all opponents except Essendon. The '52 team did however also manage a draw with the Bombers.
1937: Won against all opponents.
1925: Won against all opponents.
2007: Didn't win vs Hawthorn.
2009: Didn't win vs Carlton.
1931: Won against all opponents.
1951: Won against all opponents.
1963: Didn't win vs Melbourne and Fitzroy (the wooden spooner).
No Geelong premiership has been won in a year where all teams played each other twice during the home and away rounds.