Here's an example. I decided to build an ANZAC division for Great War Spearhead using the HaT ANZACs (wonderful figures.. and there's another example of quality). I needed some MMG crew, but the HaT figures came with no heavy weapons at all. I wrote to Ian Kay at Irregular, having found that their 20mm WW1 range seemed to be the most compatible with the HaT plastics. There are no ANZAC MMGs in their range, so Ian proceeded to sculpt some new heads for his WW1 Brit range: the result is here below. I thought this was stunning service, and a measure of quality.
While on the subject of Irregular, I decided on good advice to use the Irregular range to build my new 6mm GWSH armies. When the figures first arrived I was pretty disappointed. But judicious use of dry-brushing revealed a lot of pretty useful detail. They're not too bad.
Then, talking about service again, I regularly use Spirit Games for my Heroics and Ros 6mm figures. Phil at Spirit Games is pretty amazing: orders from Phil are almost always in my hands within 7 working days (and on the one occasion when they weren't, I had an e-mail explaining why).
Both of these suppliers have, in my book, a record of outstanding service... that's quality for my money.
Now I'm not trying to suggest that these are the only suppliers of quality in the wargames world: for example I love the qualty of the miniatures from companies like Essex and Games Workshop. I just wanted to highlight the service from Spirit Games and Irregular as great examples of quality. We need to support suppliers like this, that's our best protection against poor quality service.