Levistus's_Leviathan
5e Freelancer
If they're smart in the stat block . . . make them smart outside of the stat block. Give Vecna and Asmodeus evil schemes that trick the party for most of the movie. Have Tiamat be convinced that she's justified in trying to destroy the world because it was stolen from her by the gods. Focus on Strahd's obsession with Tatyana and how his obsession with her is impacting his entire domain of dread. I could go on.Outside of combat, yeah, they would be pretty boring. It would be difficult to portray them as anything other than big scary monsters, no matter how smart they are in their stat blocks.
Interesting fight scenes + interesting personalities. Sounds like the recipe for a good movie villain. The Tarrasque would be boring as a villain just like Godzilla would be. But Tiamat, Vecna, Orcus, Asmodeus, Lolth, Acererak, and maybe Demogorgon could be good movie villains.