Yeah, Blair Witch was shit, but are you considering 1999 to be classic? Anyway, Cloverfield, Paranormal Activity, REC, VHS are all modern, better versions of that genre.
I'd put Scream up there in the best slasher category - clever twist at the end too.
But that's kinda my point. New film styles and subgenres are pushing horror forward. SFX are ever improving. Realism is at an almost chilling level. Yes, classics are classics, but that's not at the preclusion of anything new. As I mentioned films like Martrys, Antichrist, Irreversible, Dogtooth (the last 2 are perhaps more thrillers than horrors) are redefining the genre for me and becoming their own classics.
BTW, The film Antichrist has nothing to do with the devil incarnate, incase that was why you mentioned The Omen.