This is hard so say. There are so many unique flavors of disappointing in just the last 15 years.
-2016 sucked because our hopes for a SB seemed to die with the injury to Earl Thomas.
-2008 sucked not just because we were bad but because of the multitude of FA signings and "win now" type decisions Ruskell had made that year - we seemed doomed not just for that season but for several to come.
-As an optimist, 2006 was more exciting than disappointing. In hindsight, there was much less to be optimistic about. However, at the time, we were still surviving the season and it seemed possible many of our deficiencies, like the struggles of Hass and SA, could possibly subside as the season went on. As bad as things really were, we were catching enough breaks to keep going.
-2004 sucked because of the greater expectations established in 2003 and how those expectations were undone by maddening blown leads.
-I don't care how it felt in the end. 2014 was awesome. If you're going to make a SB and lose, the kind of incredible game we had was the way to do it.
Ultimately, I'm going 2008. It's kind of a miracle they didn't doom us to more than two losing seasons with how 2008 went down.