I've never been very familiar with zombie films (or zombies in general), so I decided to start off with the film that is remembered for starting the zombie phenomenon: George A. Romero's Night of the Living Dead.
I obviously knew prior to viewing that this was a horror film, but I had no idea what I was in for. It's gruesome, violent and probably the most disturbing film I've ever seen. It's no surprise that it's still considered to be a masterpiece among the horror genre.
While the film contains a great cast (Duane Jones, Judith O'Dea and Karl Hardman, just to name a few), the main thing that makes it unique is Romero's direction and the special effects. The film doesn't necessarily contain "constant gore", but during the few scenes that it does, it's terrifying. I'll admit I had to cover my eyes several times, particularly during the last half.
Overall, Night of the Living Dead deserves to be ranked with the greatest horror films of all time. For anyone interested, TCM is airing it on Halloween night at 8:00PM EST.