What makes zombies and certain ghosts so creepy is that they are unrelenting, unwaivering, set out to kill you. Unlike all of the other monsters mentioned above they don't have a certain agenda, they don't have any forgiving traits. Vampires have to feed of humans, werewolfs are turned into beasts whether they like it or not. Aliens might be mean, but so are we to "lesser beings". Movies like Killer Tomatos or other movies like it are usually more fun than scary. Movies with "killers" are similar to zombies in that characters like Mike Myers and Jason are unrelenting, only after killing people and pretty dang difficult to kill. But they're only one, and the zombies are usually thousands or more. There is just something slightly more scary about humanity being screwed, rather than just some poor group of friends camping in the woods.
A great example of how this can turn out is Ringu vs the american remake of Ringu. Sure, the remake is creepy in that it throws scary things in your face in a typically american oogah-boogah way to scare (like Dead Space or Fear), but the original is alot more creepy in that Sadako is completely unforgiving. In the remake she is portraited as a misunderstood and abused little girl, seeking revenge. In the original she is pure evil, who will exact death on your whether you're nice or not. We don't need to feel pity or shock, just regular nervous stress is alot scarier in my opinion, and this is a subject I could expand alot more on in a post on it's own. I probably will some day (here's for promising ponies).
Another thing that makes me enjoy zombie movies so much is that to date I haven't seen a single zombie movie that wasn't entertaining, except perhaps House of the Dead by Uwe Boll (surprise, surprise). But there are of course a couple of zombie movies that are better than others, rarely because of the story or effects, more often because it includes a couple of characters that are just awesome. One such movie, or series rather, is the Re-Animator series.
It is very loosely based on Re-Animator written by H.P Lovecraft, where the only resemblance really is the names and the fact that they re-animate dead people. It's difficult to say what exactly is so good about the re-animator series. The characters, as mentioned probably the most important part of a good zombie-movie, are great. Jeffrey Combs does a surperb Herbert West, and there are usually a couple of interesting zombies as well. Combs manages to portrait a scientist who puts himself in really shitty situations, but loves it because he is so fascinated with being able to bring dead people back to life. He has a casual disregard for the danger and crazyness he is putting others and himself into. He never loses confidence in his work, never finds himself in a situation he can't handle (even when he is in situations he shouldn't be able to handle) and has an arrogant attitude towards everything and everyone that is just lovely. He is the epitome of the crazy scientist in that he gives you the illusion that he is sane, but he clearly can't be. It is without a doubt he who carries the biggest load of awesome in the Re-Animator series.
The movies also mixes humor and horror in a way that few other movies succeed with. Another good example would be Evil Dead 3, but that movie has alot more humor than it has horror. Although the Re-Animator series aren't scary, there is a suspense of danger in the air at all times since West handles the zombies so carelessly. You never really know who and what will die, or where it will happen, it seems like West has a zombie tucked up his sleeve at any moment. The feeling that there could be a zombie under any sofa and in any closet puts you on edge throughout the movie, and the comic relief never takes that away or becomes "too much". Nonetheless, Re-Animator doesn't take zombies very seriously, which generally has been the case with more recent zombie movies like Dawn of the Dead, Day of the Dead, the Tv-series Walking Dead and so on. Although I enjoy those movies alot too (I really liked the Dawn of the Dead remake for instance), I do feel like the non-serious zombie movies are they're own genre, and should be treated that way.
The intro of the movies are splendid in themselves, although the intro song is basically a complete
Bride of Re-Animator at You Tube.
If you're looking for a good couple of zombie movies to watch I couldn't recommend this series more.