I wasn't at Gilby's game at Cal, so for me the absolute worst lack of effort, team giving up, embarassement, lack of ranch dressing, was the 41-0 Stanford game in the rain in 2010. I don't think we(we!) got 100 yds of offense until LSJU put in the reserves in the 4th qtr. But after wanting Sark fired after the Nebraska game, I actually wasn't disappointed because I thought it would hasten Sark's exit. But then the Holiday Bowl happened.
I'm too young to remember the details of the eventual Rose Bowl Champs loosing to UCLA 31-0 in 1981. So for me the two biggest surprising plungers were Miami in 2001, and one that I am surprised is not getting more mentions: Nebraska in 1998.
A top 10 team gets plungered 55-7. Brock asks the Nebraska linemen to stop hitting him so hard. Gerald Harris gets a collapsed lung. Nebraska's mystique was partly built on being undefeated against non-league teams since 1991. So we (we!) took that as Nebraska could beat anyone in Lincoln, except us. So we were reportedly the first team ever to face a #2 Nebraska team in Lincoln and not only not be intimidated, but actually be too loose. I thought we had a real team that year, and loosing by 48 when you're ranked in the top 10 qualifies as both a plungering and as a disappointment.