I love monster movies. Oh, I'll sometimes watch horror movies with human antagonists; some of them (Silence of the Lambs springs to mind) are really quite good. But as a rule, I prefer actual monsters. I like watching people deal with the strange and unknown - and in some ways, the stranger the better.
I've mentioned before that I love zombies, so the fact that zombie films are thick on the ground these days is a great pleasure to me. But I'm good with vampires, too - though (while I understand the temptation to make them heroic), I think in most cases they work better as monsters than heroes. I'd love to see some well-done werewolf films; most werewolf films, even if they aren't completely awful, don't do anywhere near as much with the werewolf-as-a-monster as I think they could. I'd have said the same thing about ghosts, except that Japanese cinema has pretty well fixed that (c.f. The Ring, Pulse, Dark Water, etc.). I'm even seeing some witches (Drag Me To Hell, as a loose example), and though they're technically human, they at least have supernatural powers at their command.
But all of those are half-human, or once-human, monsters. What I haven't seen as much of, in the last decade or so, is monsters who are just... monsters. Oh, there are some - the giant beast of Cloverfield (and all its little minions), the mind-controlling slugs of Slither, the mutated fish from The Host. Most of what is out there... are either remakes or older films. The Blob, Godzilla, The Thing (either version), the giant ants from Them, the autonomous swords of Screamers, the mind-controlling parasites of The Puppet Masters, even the velociraptors of Jurassic Park (and the tyrannosaurus rex, for that matter). Also, it occurs to me that I'm neglecting two of the classic franchises: Predator and Alien, both of which feature strange and fearsome beasties.
I'd really, really like to see more films with these kinds of monsters. And, yes, that's a lot of different kinds of monsters. That's pretty much my point.
So, what's your favorite kind of monster? What makes it scary for you? Alternatively, what good (or even decent) monster movies am I missing out on? Or, y'know, feel free to head off on a tangent in the comments.
 I'd note that the Ginger Snaps series, especially the second film, is a good example of treating the werewolf itself in new and interesting ways.
 I saw this film when I was quite young, and it scared me silly - though not quite as much as The Swarm. Having watched Them again recently, I'm pleasantly surprised by how well it holds up. Yes, the special effects are extremely dated - but the movie relied more on dialogue, pacing, and presentation than special effects anyway. That particular trick where you heard the giant ants before you saw them remains creepily effective.
 Despite being nominally science fiction, this movie remains creepier than most actual horror films. The sequel, however, was... disappointing.