I've used Oxbow, National Geographic, and KMS.
Oxbow was alright and they liked it, but I was looking to buy larger bags so I went and got KMS. The chins ate both of them but they wasted a lot. KMS also came in really long straw-like pieces that I had to cut up in order to give it to them.
I bought Nat. Geo. just to see how they felt about it. They didn't seem pleased with it.
After all of that, I bought Small Pet Select. Best hay ever... I am so happy with it that I will not be buying any other brand for timothy hay. It's the greenest that I've seen, smells great, and the chins don't waste any. They eat every bit that I give them and it's amazing. They have a lot of sales/coupons as well. I would definitely recommend you try it. :]