It's the Creature From The Black Lagoon for me.
Even though in many ways it's a re-telling of the King Kong style 'beauty and the beast' tragedy, there's something about the film that has resonated with me since I saw it as a child.
The strangely beautiful Gill Man design, the gorgeous Julia Adams, powerful performances by all cast members, and an exotic locale. Add to that some truly beautiful imagery such as the underwater 'ballet' between the creature and Adams, and to me it's just a film that transcends its B-movie origins.
The Revenge of the Creature is also a favorite.
I think the 'Creature' films were a nice throwback to old style monster mayhem that had been largely replaced by the more sci-fi nuclear terrors of Godzilla, Tarantula, Them! and the rest of the fifties pack.
Famous Monsters recently had a great retrospective on the Creature trilogy that is well worth reading.