In all honesty, the show was at it's worst in 2010. The sheer amount of horrendous episodes is more of that than other years the series was around. A Pal For Gary, Keep Bikini Bottom Beautiful, One Coarse Meal, The Play's the Thing, Rodeo Daze, Summer Job, The Clash of Triton, Grandma's Secret Recipe, Yours, Mine and Mine, You Don't Know Sponge, etc.
While the series did start to go downhill around 2008, 2010 was when it hit rock bottom.