I only use Schwalbe and Continental tubes. Both of those brands individually test their tubes and I've never had a bad one out of the box. HOWEVER -

I bought four Schwalbe tubes last year and the first two had leaky valves. I bought a pack of the valves linked below and compared them under a microscope. The difference in quality was obvious. The rubber seals were larger and more even on the new ones, the metal parts are better material and more finely produced, even the knurling on the knob was more even. My theory is that Schwalbe tests their tubes first, and then installs the cheapest valves they can buy.

I still use mostly Schwalbe tubes, but now I swap out the presta valve automatically, and keep a supply of these valves. There are a lot of different ones for sale but I can vouch for these.

Presta Valve Core - 10 Pack