Who would win a fight a triceratops or a megalosaurus?

There are examples of damage to triceratops done by a t-rex attacking from in front. Examination of the damage shows that the triceratops continued to grow more bone after the conflict, so the triceratops survived the attack. This implies that a triceratops could beat a t-rex.

On the other hand, there have been fossilized dung samples which show that T-rex ate triceratops. This may mean that triceratops was hunted by T-rex, but since T-rex is a scavenger, this could just mean they sometimes ate an already dead triceratops.

So, in answer to your question, we have direct reason to believe that a triceratops would sometimes win, and little *direct* evidence in the other direction. Given the size, armament, et cetera, it seems likely that sometimes one would win, sometimes the other, with the odds probably going to the T-rex.