In my opinion, the most underrated Scheme implementation would be Sagittarius. It's a solid R6RS Scheme with clean code and really good SRFI support. The author also makes implementation agnostic libraries in R6RS (including an FFI wrapper).
>>3
I kind of lump together those Scheme implementations that I never tried even though they all seem very good: Sagittarius, Cyclone and Larceny. I know they're unrelated but they're like new kids on the block to me.