My vote is for...Lord of the Rings--The Two Towers!

I know many might not think this a monster movie, but to my mind it
does a masterful job of realizing a fictional world in which unreal
creatures, some quite monstrous prowl with regularity. The Orcs are
tremendous creations and the Ringwraiths on Dragons are great.

More traditionally, Eight-Legged Freaks is better on video than in
the theatre. I enjoyed it quite a lot, despite some severe
underacting by the lead. The critters are more convincing on the
smaller screen.

Dean