Well if my love of film has taught me anything it's that horror really encompasses a wide variety of cinema and the definition of horror has changed over time and is indeed still changing.
In terms of horror comedy I'd say evil dead 2.
In terms of psychological horror I'd say The Shining.
In terms of classic black and white horror I'd say the thing from another world (arguably, it does rely on tense human drama and there are several shock moments, not to mention the chilling ending) or Night of the living dead.
In terms of serious modern age horror, the kind that relys on intense violence, sickening camera cuts, shocks and serious human drama I'd say 28 Days Later.
In terms of a complete splatterfest I once watched one called "Dead and breakfast", that was pretty gory and bloody (But in a good way).