Okay, I haven't logged in here in YEARS, but I love the Darkover books (which I mostly read back in the eighties) and am happy to make some recommendations...
My favorite books in the series are the ones that take place after the Terrans return to Darkover... and before MZB ended up using ghostwriters for her last few "authored" books. (The earlier-set books, the "Ages of Chaos" ones, are pretty much straight-up medievelesque fantasy, missing the more SFish tropes, and - especially - the Terran technology vs. Darkovan "primitive feudal society" culture clash that really brings out the best in the series.) I recommend reading them based on their internal chronology... this is the essential sequence:
The Spell Sword =>
The Forbidden Tower
The Shattered Chain =>
optional: City of Sorcery, Star of Danger, The Winds of Darkover
The Bloody Sun (the revised version!)
optional: The Planet Savers
The Heritage of Hastur =>
Sharra's Exile (not the earlier embryonic version, The Sword of Aldones)
The World Wreckers
As far as which books are the best, strong cases can be made for direct sequel pairs The Spell Sword/The Forbidden Tower, The Shattered Chain/Thendara House, and The Heritage of Hastur/Sharra's Exile. All are excellent: the late seventies found Bradley at the height of her creative powers, and adding explorations of previously taboo subjects like feminism and homosexuality to the fascinating Darkovan/Terran brew.
But heck, most of the Darkover books are pretty good, and the size and detail of the created universe they represent is really impressive. Enjoy!