For me it was Rygar. I LOVED this game on the NES when I was growing up. Just like Keatah, I never knew it was an arcade port until I was an adult and discovered it through emulation. For the record, I think the NES port is way better to me.
Also, and this relates to a couple of threads in the Emulation forum, I never could beat Rygar on the NES back in the 80's. It was and is one hard ass game to beat! It wasn't until the modern era that I was able to beat it on an emulator using save states. Yes I know it's cheating, but I always wondered what happened at the end of this game so it was totally worth it to use save states to beat it. So tha'ts one of the advantages of emulation that Keatah so eloquently described in the Emulation forums.