Loire chateaux that competes w the big boys?

We stumbled on Cheverny just driving around and enjoyed it a lot. Azay le Rideau is quite picturesque and wan't crowded when we were there, but then neither were the bigs. Chambord and Chennenceux were both uncrowded when we visited one May. We also enjoyed Sully and Chateneuf en Auxois but they are in Burgundy.

I'm not sure what you consider the biggies but I'll assume you mean Chenonceau, Chambord, Villandry, and Cheverny. And I don't know which chateaux are on the RS list. I stayed in Azay le Rideau and enjoyed that chateau but it may be considered one of the biggies because it is on a lot of the Loire Valley tours. I was there in July 2012 and it was not particularly crowded - either the town or the chateau. My favorites were the two in Amboise, the royal chateau and the smaller Clos Luce, both for the Leonardo da Vinci/Francis I connection. Neither of them were crowded when I was there but that was quite a long time ago (like 15 years ago) so it's hard to say about now.

I visited Clos Luce and Amboise on a tour in September 2012, and neither one was crowded. I enjoyed them both.

Lesser know chateaux in the Loire Valley (they're probably well known to some), but how about:
Valencay
Usse
Langeais
Loches
Chaumont (better known but nobody has mentioned it yet)

I liked Château de Montrésor some 20km east of Loches. And Château du Lude along Le Loir too, some 50km north-west of Tours. Both worth a visit to my opinion.
http://www.chateaudemontresor.fr/accueil.php
http://www.lelude.com/en/index.php

My wife and I greatly appreciated Chaumont-sur-Loire. If you like gardens, it has a world-class garden show that lasts all summer with more than 30 different designer gardens. We liked it better than the Chelsea Flower Show--more creative and not as crowded. The chateau itself has lots of American connections--its prime owner was a great supporter of the American Revolution and helped finance Benjamin Franklin. And this chateau is actually on the Loire, so it has splendid river views. And it's not far from Amboise.

I think Chaumont makes a good base for exploring the Loire as well because there is a bridge there that allows access to both sides of the river without the traffic of Amboise and Tours. We stayed in B&B in Ozaine on the north side, directly across from Chaumont, of the river and easily got around to all the chateaux we wanted to see. In addition to the garden show, there is a historical connection between Chaumont and Chenenseau. I like Cheverny because it is fully and lavishly furnished. There are many smaller chateaux that are interesting and not crowded. Go to the "biggies" early to avoid tour buses.

Let me add Blois to the list. It's not a fairy-tale palace like Azay, Chenonceaux, and such, and it's in a fairly big city so you have to navigate some. But it's very historic and architecturally very interesting. On each side of the courtyard is a building from a different period between the 15th and 17th centuries. Each building has different kinds of exhibits. Check it out.

Other less prominent places we liked include Clos Luce (Leonardo's home) and Loches (chateau inside medieval walls, and even some Roman stuff in front of the chapel).

Another one, farther off the beaten track, would be Valencay, where Talleyrand lived.