The one that comes straight to mind is King Ghidorah. If you're going off the Monsterverse version, how would Venom interact with Ghidorah as its one body with 3 individual minds?
Something to remember about Venom is that the emotions of the host influence the Symbiote, so other pure good monsters like Mothra could instead work together with the Symbiote for the good. Then again, the Symbiote is an alien lifeform and therefore disturbing the balance of nature so maybe not.
Also, I just wanted to mention that Venom with SpaceGodzilla as its host would be the most terrifying thing ever.