I haven't gotten very far into desynthing yet. I choose LTW and GSM because a post from a long time ago listed those two as good sources of demimateria. (But as others have pointed out, which classes are good keeps changing from patch to patch, so that's probably not even accurate anymore.) As my third, I picked CUL simply because there are so many fish that have no other use besides desynth, and I like to have a use for everything I gather.