I had to use multiple votes. But if I had had to use one, it would have been the Epic Level Handbook (although I also voted for AEG, DDG, Psionics Handbook, Stronghold Builder's Guidebook and Book of Challenges). When I read the Paladin advancement table in that, I actually laughed out loud in the gaming store.
However, if I were to vote for the most disappointing, it would be the DDG. There have been such advancements since 1980 in how games handle religion, it was very disappointing to see all that text wasted on how gods do in combat while all kinds of questions about how their churches work, etc. remained unanswered.
I voted for AEG because I bought it and felt ripped off. I suppose I wouldn't hate the book if it had cost me less than $15 instead of the $25+ it cost me.