The best selling console of its time, the NES helped revitalize the US video game industry.
Nestopia is one of the best NES emulators available. Its open source, has had a steady update history with a long list of game compatibility and supported controllers. The only reason this emulator did not recieve a 10/10 is the out of the box netplay. Some tweaks are required if you wish to get netplay working correctly. Because there is a short list of desired netplay games however, it is still a very good out of the box emulator.
FCEUX is a cross platform NTSC and PAL NES emulator that spawned as an evolution of the original FCE Ultra emulator. The concept behind FCEUX came from merging elements of other FCE programs into the current emulator. This allowed users from multiple communities, speed running, ROM-hacking and more to use an all-encompassing emulator.
Jnes is light weight, simple NES emulator. Compared to some of the other NES emulators Jnes requires a higher minimum system requirement. For the most part its not a bad emulator but some issues can arise if your drivers are not up to date. For that reason it is ranked below FCEUX and Nestopia. Still a neat emulator though.
Getting past the funny name Nesticle is actually a pretty decent NES emulator. It hasn't seen many updates, since the source was leaked, Nesticle was discontinued. Early on however Nesticle played a big part in emulation and is still a good choice. Originally released for DOS and windows.
VirtualNES of Japanese origin is actually fairly new in comparison to the other NES emulators with a current English version. It supports full CPU and sound emulation but requires the most recent versions of DirectX.
This is a decent emulator with a lot of features, it isn't particularly fast compared to other emulators. RockNES does however support Game Genie and some other cool customizations.