So far there are Best and Worst Sci-fi series of all time, so I'm going to do my own thread.
The worst Sci-fi films in my opinion are:
5. Lost In Space (In 4 words: "Danger, Danger, Will Robinson")
4. Galaxy Quest (The only reason why I include this film, is for the obvious Star Trek rip-off that it is, although I did think it was funny)
3. Barbarella (Not so much the film, Jane Fonda)
2. Spaced Invaders (A bad film overall)
and the number one worst sci-fi film of all time:
1. Battlefield Earth (Had many things going against it: bad script, didn't follow the novel at all, the worst performance of John Travolta, ever)
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"Cry havoc! And let slip the dogs of war..."
-Chang (ST VI)