I always felt Transcend and Patriot does a good job and have good speed, never had to RMA them. AData is okay, but slightly slower for me, but probably not enough to notice. I was given an Adata class 6 that was extremely slow and I seem to break them easily, but it could be just me.
Sandisk isn't going to fast unless you get their more expensive models, but I use them too since you can find cheap deals once a while.
Patriot also make some nice flashdrives.