Fantasy: I've never seen Warriors of the Lost Kingdom, so I can't comment. About the worst that comes to mid right off, in a sword and sorcery vein, is either
Ator: the Fighting Eagle or it's sequel. And I'm too lazy to go to IMDB and look it up.

The giant spider... oh my god.
SciFi has to be
Screamers. God, it was spitting bad, where you walk out of the theater spitting to get the bad taste outta your mouth. A close second is Aliens 3. Yeah, let's kill off the two nice characters from the second movie, AND let's get a director that heretofor has only done second-rate music videos.
My three all time crapazoid films have to be Mortal Kombat II (I was a very nice film, so the sequel being so bad hurt even worse), Batman and Robin (Way to kill a franchise, dude) and Screamers. Generally, I don't look at the truly low-budget films for this kind of consideration. They tried, on some level, most of them. But these.. either big budget, or an ace director, or coming on the coattails of a successful film.. they had no excuse. None, whatsoever. It's always said that no-one sets out to make a bad movie, but... sometimes you have to wonder.