The term “Pokemon emulator” is as misguiding because it sounds. Emulators, as I said are “system-dedicated” not “application-dedicated”. By this, what I mean is there's no Pokemon games specific emulator intrinsically . Instead, there are emulators made for PC to emulate the games played on devices like Game Boy, Game Boy Color, Game Boy Advance, Nintendo DS, Nintendo 3DS, Sony PSP, Xbox 360 etc. and Pokemon games are released for few of those systems. the foremost popular Pokemon games are released for Game Boy Advance (GBA), Nintendo DS (NDS) and Nintendo 3DS .
you can download emulators & roms GBA and NDS and ds emulators from:
There are not any emulators for Nintendo 3DS therefore the games released on 3DS can't be played on PC.
According to my experience the simplest emulator-
For Game Boy Advance is Visual Boy Advance
For Nintendo DS is DeSmuME
For these emulators to figure you need to load the ‘roms’ of the respective games within the emulators. e.g. Pokemon Crystal rom must be downloaded separately then opened via Visual Boy Advance. Pokemon Black rom must be downloaded and loaded in DeSmuME.