One of the best examples of decisive defensive strategies being the key to victory was in 2007. The alliance that won the championship that year was an 8th seed in a competitive division.
The alliance of 177, 190 and 987 based their strategy on slowing down the opposing alliance and making up the difference in score during the end game. This methodology paid off consistently enough for them to secure a championship win.
for more information please see:
http://www.thebluealliance.com/event/2007cmp
Other great defensive plays (and many poor ones) can be found by watching match video from 2006 when there was designated time for teams to play defense.