HOTH, though PG and IV are not too far behind it. When I was "discovering" LZ and going through their discography, just the entirety of HOTH really stuck to me, and it has since become my favorite album by them. IV has a lot of great songs, but I've heard those singles played to death and back, so it's kind of worn off it's sheen, and PG is a great double album, but I don't think I've listened to it enough to really give it the title of my favorite just quite yet, but both of those are a close tie for second.
-Marc.