Game 5 of this series vs Detroit > Game 6 of this series vs Detroit
With that said, I'll list other way, way, WAY bigger games in Celtics' history:
1957 Game 7 vs the Hawks in the Cs first NBA finals (after 6 straight years of losing in the Division Finals(3 times) or semis(also 3 times)).
1966 Game 7 vs Lakers, NBA Finals b/c it gave the Cs their 8th straight championship, a record no team will ever beat
1974 Game 7 vs Jabbar and Bucks for first post-Russell championship
1960 Game 7 vs the Hawks for the Cs first repeat NBA Championships
1962 Game 7 vs the Lakers for their 4th straight NBA Championship
1984 Game 7 vs Lakers for #15
1969 Games 6 & 7 vs Wilt and the Lakers, down 3-2 had to win both for Russell's 11th and final ring
1965 Game 7, ECF vs Wilt and the Sixers to propel them to their 7th consecutive NBA Championship
1968 ECF Games 5, 6 & 7 vs Wilt and the Sixers, first time a team came back down 3-1 in the conference finals to win the series.
1981 ECF Games 5, 6 & 7 vs the Sixers. Bird ERA first championship
1966 EC Semis vs Oscar and the Royals, Game 4 and 5 (best of 5) brink of elimination the year they won their 8th straight championship
1984 Game 7 ECSF vs Knicks
1988 Game 7 vs Hawks, ECSF
2008 Game 7 vs Hawks, first round
2008 Game 7 vs Cavs, ECSF