PostGIS is one of those rare examples of highly specialised software that is both OSS and best-in-class. Usually OSS is relegated for highly commoditized software but GIS is anything but even after all these years.
I can't really think of many others? Maybe OptaPlanner would be another candidate.
Yes, among off-the-shelf GIS databases, PostGIS is generally as good or better than anything else on the market. More capable systems exist, particularly in terms of scale and performance, but those are all bespoke.
It's definitely up there. I guess it's hard to say there is a single "best-in-class" solver because it does depend on the problem at hand but if I had to pick one it would be OptaPlanner.
I'd say it's not quite in the same category as Postgis, where the open source project is handily better than any closed source rivals. It competes well with some closed source competitors but not all of them.
I can't really think of many others? Maybe OptaPlanner would be another candidate.