The Chronicels of Deryni by Katherine Kurtz
I also enjoyed the second series (Camber of Culdi), but its doesn't crack my top five. After that the Deryni novels became way to depressing for me to read. I made it through the third but didn't finish the fourth.
Hitchhikers Guid to the Galaxy and Hobbit/LOTR with the above are my three most reread series.