Not all horror comics, even old ones, are created equal. It's all about the cover art.
If you stick to the books on the two
lists linked above, you'll do well -- these are the titles with the most outrageous cover art, featuring severed heads or limbs, horrendous scenes of torture or suffering.
It ain't pretty, but it's pretty effective. The nice thing is, these books are still under-valued compared to Silver Age comic books.
I'm slowly
putting together a collection of these great titles, and I strongly recommend you do the same.