I'm a big fan of horror movies and have seen quite a few over the years, but I'm afraid I'm going to have to go with the obvious answer. I love the original Halloween. It's not only my favorite horror movie but it's in my top 20 of favorite movies overall. The atmosphere of the movie is awesome and the music is second to none. My second favorite is the original Dawn of the Dead. I think it's the best movie Romero has made. The original Texas Chainsaw Massacre is way up on the list as well.
As far as obscure movies, last year I discovered Don't Go In the House
. I thought it was pretty disturbing. If you like slasher films Just Before Dawn
is an obscure but really good one.