Definitely Ghost Opera by Kamelot. Even though I became a fan after The Black Halo was released, I loved the album and hoped they'd come back with something awesome like it, but Ghost Opera is a borefest.
Godsmack - IV: Even though Sully Erna's lyrics were still anti-social angst, Faceless was a kick ass metal album that showed that the guys weren't fucking around, especially with new drummer Shannon Larkin. Then they followed it up by going backwards to a muddy produced and recycled mess.
Dream Theater - Systematic Chaos: My first real time Dream Theater album, and although it did get several spins, it lost it's steam very quickly until I only listened to In The Presence Of Enemies, but then that lost its steam as well. I never liked The Dark Master section, and Prophets of War was atrocious. The only part of Ministry I liked was the instrumental section. I don't mind Constant Motion or The Dark Eternal Night, but I just have no desire to listen to them. Forsaken was pretty bad too, and Repentance isn't a multi listener.
The Offspring - That Shitty New One: By far my biggest disappointment is The Offspring. My first favorite band and obsession. I've been following them officially since Ixnay On The Hombre, and although they started decreasing in quality, I still enjoyed them greatly. Even Splinter had some good tunes. For their newest, they completely changed their style while ripping themselves off in the process, and became just another pop punk emo trend. I've claimed that Mr. Microbiology, Dexter Holland is smarter than that, but his wits aren't in question as he knows what'll make him more money. The Offspring have serious potential and have always been more than just an average punk band, with traces of classic metal thrown in, but all that fame and fortune has gone to their heads.