I think the most important quality is the fit. I mean, what good is a radiator if it doesn't fit right? Radiators can have several factors that could cause it too fit slightly different such as the dimensions, location of the mounts, hose connections, core thickness, and cooler fittings just to name a few. These are always a good thing to check before you buy a radiator.
Another quality difference is the plastic quality and the aluminum quality that is used. The quality of the materials can cause a radiator to last for years to come, or lead to its demise after only a year. The way most radiators go out is the fact that the plastic cracks, or the header leaks in the corner. Using higher quality materials can drastically change the life of the radiator. however, you still have to remember that wear and tear always happens slower by taking care of the vehicle.
The final major factor of quality in a radiator is the core thickness. If the original radiator has a 1.25" core , you don't want to replace it with one that only has a 1" core. Several radiator companies will reduce the size of the core to cut costs for a cheaper radiator. This way seem fine at first, but it can cause overheating in the future, and even help the core fail.
With that said, you know what to look for in a good radiator. Just because a radiator is made in China (most of them are) does not necessarily mean it is of low quality. Every brand has certain radiators that don't work as well, while others that they sell are some of the best out there. As far as actual good brands, I would say some of the more consistent brands with high rates of good parts are GPD, Champion, NRF, Koyo, and Tong Yang. The only one out of these brands not made in China is NRF, which are manufactured in India or Taiwan with its corporate office in the Netherlands. GPD and Champion are both American owned and run, while Koyo and Tong Yang are both Chinese owned and run. Some brands that we have seen a decline in quality are CSF and DixinUSA. We have ran into several cases lately where these brands didn't include the necessary hardware, or the cores fail early on. Radiator brands which we have seen the lowest quality, or highest amount of problems are Silla and Reach. Silla radiators we have found are of low quality materials and have a high fail rate. We have had several cases in times past when Reach radiators had a failed transmission cooler, causing coolant and transmission fluid to mix.
What it all boils down to is that every brand has its ups and downs. The best way to get the best radiator is to go by the guidelines expressed above and to get the best product, or trust the radiator expert near you!