Another vote for Prydain. A true classic series. I re-read them in my early twenties, and I thought they held up just fine. Any fan of fantasy who has not already read them should do so immediately.
I tried Fionavar but gave up after the first volume. As far as I can remember, I either never much liked or never much believed in the world the books took me to.
Can’t say anything about any of the others.