I don't have a favorite quality brand. I've never had one of the really expensive saddles or bought one new. I had a used Stubben that I loved, but it didn't fit Bay well and I traded it for the Wintec I have now. I've also had one of the supposedly cheap Argentine saddles, and it also held up pretty well for my use.
I know you said you wanted to buy a quality used one... Passier, Crosby, Stubben, are all good brands. However, I can't go without recommending a new Wintec synthetic. They aren't more expensive than a lightly used leather saddle (I paid ~$400). I really like mine, it's really light, has a really soft seat and soft panels underneath, and they have interchangeable gullets to fit the wither of your horse. It's still relatively new, but holding up really well.
The thing to most look for in a used English saddle (besides a broken tree) is that the padding panels on the underside are still soft. You should be able to depress it with your fingers. Over time, they get stiffer and stiffer. Some of the cheaper saddles start out pretty stiff, the Wintec ones are particularly soft. Older saddles can be repadded, it's about a $100 repair job. If you go to a saddle shop, poke and feel a few of them.
I've seen lots of used Stubbens and I knew it was a good brand but it's always good to hear about them from someone who has actually used one. I'm not finding many used Wintecs and the new ones I've seen were $1000.
I have a kid size Pessoa that I'm not using anymore, I was going to try to find someone interested in a trade. The same saddle new at Dover is $800. People are crazy to spend that much on a kid saddle that the kid is going to grow out of. I bought mine new but I paid $500 or so for it about 6 years ago.