I also have a Strymon Big Sky which is my go-to reverb for the Prophet 12, it's amazing, seems to fill out the P12-sound with a lush richness. Maybe the reason i love my P12 so much is more the Bigsky's doing than the synth itself.
While it can do great classic room/spring/plate/hall sounds, i tend to mostly use the swell, bloom, cloud & shimmer algorythms for sounddesign, and use a second reverb unit for other sounds in the mix that need a more traditional spatial ambiance. (lately those are usually plugins from the Valhalla-series)