I couldn't come up with a list of the best books. But I did come up with books I've read more than 10 times each. Here are some of them.
Christopher Stasheff's A Wizard in Rhyme set. There are 8 starting with Her Majesty's Wizard. I have read five of them more than 10 times each.
David Weber's Honor Harrington series. There are 12 books in this series. I have read 10 of them more than 10 times apiece.
Gordon Dickson's Dragon and the George series has nine books in it. I have read at least 5 of them more than 10 times and I read one (The Dragon, the Earl, and the Troll) at least once every December as well.
Elizabeth Moon's Deed of Paksenarrion has three books in it. There are others attached to the series but they stink. The three in this omnibus are Sheepfarmer's Daughter, Divided Allegiance, and Oath of Gold. You can find reviews and lists of other books here. There's no review for The Speed of Dark, which I've only read once, but it's a fascinating near future book about autism, a disability which Moon's son suffers from.
Patricia Wrede has four books in her Dragon Series. Reviews are here. I have read all of these at least ten times. Also, they are suitable and accessible for younger readers.
PC Cast's Goddess by Mistake is a fun and fanciful book. It is set in a parallel universe and is funny. It's even better if you've had a chance to hear PC read it to you.
Robert Aspirin wrote the Phule series. The best are the first two: Phule's Company and Phule's Paradise. I've read them way more than ten times. The others in the series weren't as good, but were still very readable.
Mercedes Lackey wrote a Valdemar Series which has more than 23 books in it. I've read all of them at least twice. I'd say I've only read six of them more than 10 times. Those are the original three and the Gryphon set.
Lackey also has a Bardic Voices series. I've read the first four multiple times. When the fifth one started on the premise of women being murdered, I just couldn't keep reading.