Blake Stone is often forgotten about given that it came out just one month before Doom, but despite that it had a strong showing for what little time it had before Doom hit the scene and obliterated the competition as far as first person shooters were concerned. That said, it's freaking hard and has some of the most ridiculous secrets-within-secrets chains I've ever seen. It also has some rather unique mechanics to help keep things interesting compared to other similar titles.