Oh, man -- fantasy is so filled with series, and Del Rey (especially during Judy's time) was all about non-standlones. Pern, Shanara, and Xanth were all from her time, and are still going. But if you haven't read it, The Princess Bride really is a fantastic novel, one that offers a lot more than the (fantastic) movie it was adapted into.

Other standalone fantasy that I've loved includes Charles de Lint's The Little Country and Emma Bull's War for the Oaks.

Sadly, most of that 20% sf/f I read tends to be SF these days, with the exception of some ongoing series like Steve Brust's books. One of the few recent standalones I can recall enjoying would be Race the Sands by Sarah Beth Durst (who mostly writes YA, but this one's definitely not).