Here's my brief opinions on what I've seen:
I have to say I like the remake of The Thing better than the original. Just watched that again a couple weeks ago and everyone in the room agreed the effects still stand up today.
I like both versions of The Fly for different reasons. The original was pretty creepy but the remake had a more interesting story to me. Jeff Goldblum being nearly naked didn't hurt it for me either ;D.
I've only seen part of The Blob and not the remake so can't comment there. I enjoyed what I saw though.
Body Snatchers is about the same feelings as The Fly (although Jeff Goldblum wasn't naked). Body Snatchers was a good creepy movie with an ending that I wasn't expecting. Donald Sutherland was great.
I prefer the original Dawn of the Dead to the remake. The remake is too much like most modern horror movies, going more for the scare that makes you jump or gross you out over plot. The original is much more re-watchable.
Haven't seen the Living Dead remake. The original is perfect so I never felt the urge to see a remake.
Cat People I've only seen the remake. Didn't make much of an impression.
The original Hills Have Eyes is a lot better than the remake. Didn't like the remake for same reasons as Dawn of the Dead. However, the remake was a bit scarier as far as makeup effects go!
I refuse to see the remake of Last House on the Left. I love the original and it was done perfectly. I've heard how the remake goes and just have no interest in it.
Another case where I like both for different reasons- The Ring and Ringu. Ringu is great as a creepy, suspenseful movie. The Ring went a bit more for effects-related scares. So both were scary for different reasons. That seems to happen with most foreign movies that get remade for an American audience (like Dark Water and The Grudge).
I was never a fan of the original Halloween. I appreciate it for what it was and enjoy parts of it but it just bores me. The remake didn't really have boring parts but just came off as a typical modern slasher movie.
I also adore the original Texas Chainsaw Massacre. I've seen it a few times. I watched the remake once and hated it. The acting was horrible and again it was going for the cheap scare.
The original Amityville Horror was a lot creepier. The remake relied too much on CGI and the ending was just awful. The book scared me a lot more than the movie as a kid.
Friday the 13th I've only seen the original. Not sure I want to see the remake.
2000 Maniacs only seen the original and I liked it (might have seen that the same night I saw The Crazies).
Willard I've only seen the remake because I'm a Crispin Glover fan. It wasn't as bad as everyone said. Crispin Glover was perfect for that role.
Mummy & King Kong I've only seen the remakes. I've seen bits of the original King Kong though. They weren't bad movies.
Little Shop of Horrors I only saw the remake. I own the original (and the Rifftrax) so I'll get around to watching it eventually!
I also have mixed feelings on the Elm Street remake. I love the originals. I might check it out on Netflix or if a group of my friends goes to the theater.
Whew!