"Rosemary's Baby" is one of the most sophisticated and artistic horror movies ever, if not the most. IMO, it's the last GOOD horror movie. At least, in my view, horror movies changed after that, going in for "Halloween" type slasher junk. I'm sure there have been some other decent ones in recent years, maybe "The Sixth Sense" and "The Others," though I haven't seen them, but from what I've read they are derivative and not as original as "Rosemary's Baby," hence I wouldn't rank them on the same level ... "The Blair Witch Project" was certainly original, but I wouldn't care to see that one a second time, unlike "Rosemary's Baby," which continues to be interesting after multiple viewings.
Everything I've seen of Polanski's work is masterful and classic.
I guess "The Exorcist" and "The Omen" came out after Rosemary's Baby, but again, I don't find either one as good, althought the novel of "The Exorcist" is far superior to the shlocky written "Rosemary's Baby" -- RB being one instance where I think the film was better than the book.
As an aside, I was paging through the book "Big Lou" (when I finally received it shortly after placing a second order for it at amazon) and one of the first things to catch my eye was an error that surprised me. The author names the charming British actor Maurice Evans (remember him from "Bewitched"?) as playing Roman, when of course he played Rosemary's older male friend and confidant. I was rather surprised by the error. Both actors gave superb performances in RB.