For over-the-top excess: The Breakers

For actual livable charm: Kingscote

For dramatic, slightly irreverent interpretation of the lifestyle: Astors' Beechwood

To me, the Breakers is the most spectacular. And it has gorgeous views of the ocean.

If there's only one you can visit, I'd go to The Breakers.

I think the Marble House & Rosecliff are pretty nice too (these 2 are also along the Cliff Walk with great view of the ocean.)

I agree with those who've said if they could only choose one it would be The Breakers. The last time we were there we toured others that we enjoyed very much. But, to me, The Breakers most represents the place and the era.

The Breakers for sheer over the top opulance, but it is a guided tour.

If you don't like guided tours, and want to go at your own pace, check out The Elms or Marble House, they both use I-pod type devices that you use at your own pace making the tour as involved as you want it.

To see period actors playing the part of the Family, Staff, etc. then Astor's Beechwood.
http://astorsbeechwood.com/

To see how somebody with $$$ lived up until recently, go to Rough Point. Doris Duke's manison where she lived until her death in 1993.
They are currently showcasing her animal sculptures and art.
http://www.newportrestoration.com/roughPOINT/estate.html

So, you can see, there is really no "best" it all depends on what you like....

The Breakers. It's unimaginably impressive! Stunning views from the grounds, too.