As much as I hate his more recent films (post Land of the Dead) and the stupid zombie-fad we're stuck in, Romero deserves to be mentioned amongst the higher echelon of horror directors for what he's accomplished in the past.
I'm also fairly partial to Day of the Dead, which seems to have picked up a bit more respect since its release. The zombies here are less the focal point than the unsettling human drama playing out between the survivors. If Dawn is a depiction of mid-apocalypse, then Day is the first true post-apocalyptic film in Romero's series. Remnants of the human race are trapped underground, coming into conflict with each other more than the undead. I get the sense when I'm watching it that the actors haven't slept in days and it makes for a very tense, engaging film.
As much as I enjoy these two zombie flicks, I wouldn't have felt Romero worthy of this list if they were his only accomplishments. What really elevates Romero for me is his 1976 take on the vampire - Martin.
The film tells the story of a young man who believes himself to be an ancient vampire. Romero presents a very good "is he or isn't he?" narrative,offering evidence for both possibilities - the alternative being that he's just crazy. Either way, he still stalks and subdues victims and ultimately drinks there blood. Martin is a tragic and twisted antagonist and his story, set against the backdrop of a failing industrial town in Pennsylvania, results in one of the better vampire films ever made. (Lofty as that praise might seem, 99% of movies about vampires, werewolves, zombies and miscellaneous ghouls are downright terrible).