hmm... wow, a lot of people on here hate some of the stuff I really liked. As a big fan of wilderness adventures back in my DM days, I rather liked the WSG... granted, it could have been done better, but it did have some useful stuff in it.
Maztica and the Horde... good book ideas, not so good gaming ideas. I did make use of Maztica.. not the Aztec lands, but the vaguely mentioned lands north of them (the counterpart to N. America).. basically, I had the PCs running around in pseudo-Iriquois lands, with a few Vinland viking type colonies for fun. The Horde.. never did make use of it.
I don't know if I can pick one D&D product that qualifies as the worst. As a general thing, sequels seemed to be bad. The first Undermountain set had some good stuff; the sequel was bad. The original GDQ and Slave Lords modules were great, but when TSR gussied them up in shiny new supermodule books, the add on stuff tended to be bad. And of course, there were all those 'Return to.." sequels to classic modules (KotB, ToH, etc.) that were pretty bad.