I just recently re-watched this and the remake...there's really no fair comparison.
Gregory Peck wins over Liev Schreiber, but then Peck is the better actor.
Lee Remick is far more believable than Julia Stiles who doesn't seem to convey the same earnest fear and suspicion.
David Thewlis is a good actor, but in bit parts he always seems to phone it in and David Warner was just the more believable photographer.
I mean, the 1976 The Omen is dated, but that's not a bad thing and in this case you get the sense that they were doing something fresh and really trying to frighten you...and they did.
By comparison the remake is paint by numbers and offers nothing new.
1976 is, hands down the more frightening, more dramatic, and more suspenseful film. Compared to 2006. 1976 is believable.