It boils down to a few distinct characteristics that will make a cap "quality" to you:
- material - ABS or PBT are common. Both are "quality". ABS will start to shine sooner. PBT will take much longer to develop a shine. POM is more rare. There are other difference, but you can google "PBT vs. ABS and you'll find lots of threads comparing them.
- thickness -- most people like caps thicker. 1.5mm is considered "thick", 1mm is considered "thin"
- profile -- there are many profiles. Here's a great thread on it: https://geekhack.org/index.php?topic=68550
- legend technique -- legends can be laser etched, dye sublimated, or doubleshot (2 layers of plastic). Doubleshot is typically preferred for ABS but is more difficult for PBT so dyesub is "good".
- color -- this is personal preference of course, there are no "good" colors really. But there are more color choices with ABS doubleshot than with PBT dyesub (because the legends must be darker than the rest of the key)
Keycaps have very different sound, based on material, thickness, and profile. That can also be a differentiating factor for people.
There's a thread on geekhack with links to some vendor sites:
https://geekhack.org/index.php?topic=45672.0. Some of them are now defunct but several are still operating. I have purchased caps from geekkeys.com and Originative.co with good success. I've also purchased on /r/mechmarket keysets from past groupbuys and I think that's a great option, though the coveted colorways tend to sell for higher than the groupbuy price. imsto.cn has very good pbt caps in cherry profile.
GMK is a company that has Cherry profile molds. You typically don't buy directly from them, but rather through groupbuys, or some vendors stock particular colorways. You can buy GMK Honeywell or GMK Dolch on Originative.co right now. Or as mentioned above, you find a lot on /r/mechmarket.
Hope that helps!
