Frances the Mute by The Mars Volta
How did I miss that one! Such a good album! For me, they peaked with Frances - I just did not find any of their albums after it to be as good as that, even though I own every single one of them (bought on release week in the hopes that they'd produce something equal to or better than FTM).
Genesis - Trespass
Emerson, Lake & Palmer - Tarkus (or Pictures, depends if you wanna count that as a regular album. Both are fantastic)
Haken - Visions (although it's my least fav Haken album - which really doesn't mean much...)
I thought about putting Genesis in my original post, but considering just HOW different they were between FGTR and Trespass, it didn't seem much of a fair comparison to say that their second album was great since many fans consider it their first true album since FGTR is so different from any of their earlier albums with Peter.
ELP is a good choice, though, and Haken as well. Visions is probably my favorite of their albums so far!
Nothing to do, so I checked some.
*SNIP*
I asked for *great* second albums, not every sophomore album you own!
-Marc.