I think that’s the crux of the problem, actually. It should have been a fantastic movie, but instead it was only blandly passable. I think it’s the let-down aspect that gives it that terrible reputation.
I am voting for Highlander 2.
It could have also been the fact that the film is completely incoherent, incredibly poorly shot (the editor made Michael Bay look restrained), and a slapdash effort by everyone involved in the production of the film.
Definitely Zardoz.
Highlander 2. RickJay is entirely correct on that one. There isn’t a moment in it that isn’t actively painful.
Zardoz on the other hand is fun, Fun, FUN! I, too, own it on DVD and treasure it for its glorious ridiculousness :p.
Dammit, I must hunt down that Zardoz movie. It looks like a scream. 
Another vote for Zardoz (the Agony Booth recap is hysterical, though).
I was gonna say that. I have never walked out on a movie in the theater, I have never watched only part of a movie at home – except First Knight. If a movie is truly awful, I’ll start deconstructing it; just what is it why I don’t like this offering. First Knight was just too damn dull to even find that amusing. I mean, it’s got Sean Connery, it’s got Richard Gere, it’s got Julia Ormond, it’s about King Arthur; it sounded great. But about forty-five minutes into it I snapped it off, intending to pick it up again the next evening – and never did. Two days later, back to Netflix it went.
If League had been called “Alliance of the Super-Victorians”, it would have been a much better movie, just like if Tim Burton’s PotApes had been titled “Simian World”. Alas, the existence of LXG means that it’ll be a long time before the LEG graphic novels get a proper film treatment.
Yes, I do want a Hyde-raping-Griffin-to-death scene!
To the OP, Zardoz for sure. Doesn’t mean it’s unwatchable, though.
Hmmm, for that, I might change my vote to The Avengers.
My vote is with Highlander 2… <sigh>… I’m one of the people who rather wishes this hadn’t been made, (and sometimes jokingly pretends it wasn’t)… the original Highlander is a movie that (mostly) works despite itself (and the Queen soundtrack goes a long way). The sequel is… I mean, why? Why pitch it as a sequel but completely disregard / deviate from the original… the human genetic “sport” Immortals are revealed as aliens from the planet Zeist?! Polite word fail me. 
Zardoz on the other hand doesn’t come close to the worst (IMO). It is a 70’s psychedelic optical art piece, and yes it’s probably better watched stoned (and certainly may have been written while under the influence), but it also has some interesting ideas and concepts around the Eternals; their eugenics experiments, own personal decadence and decay, and eventual downfall. (Must find a copy)
Zardoz gets points for sheer weirdness. LXG is just stupid on an ordinary big budget level. I haven’t seen Avengers so I’ve gotta go with Highlander 2, which is unwatchable.
Consider yourself lucky! Not only did they make Highlander 2, they made Highlander 3 and Highlander 4.
At least Sean Connery had the good sense not to dig himself any deeper into that hole. Highlander 2 remains the low point of his career, IMO.
It’s been a couple decades since I saw Robin and Marion, but I’ve seen it twice and it’s a good film. My memory, in fact, has it rated as very good.