So this got me thinking, which formats have sold the most hardware units capable of playing that format. I only included hardware capable of directly playing the original physical formats, so no digital distribution such as PS1 Classics on PSP or Virtual Console on Wii. Also no converters such as the ones for Master System on Mega Drive and Game Gear, Super Game Boy on SNES, or Game Boy Player on GameCube. I'd like to include those, but there aren't any sales numbers, I doubt they sold huge numbers anyway.
PS1 - 300.39M
GBY - 197.78M
GBA - 193.13M
PS2 - ~156M
NDS - 144.59M
GBC - ~119M
NGC - 106.38M
GBA is still selling (DS Lite), but probably not enough to pass GBY, maybe if sales for Game Boy Player were known it may pass pass 200M. GBY is probably past 200M if Super Game Boy and Game Boy Player sales were known.
NDS looks assured to pass both GBA and GBY with 3DS retaining compatibility with it along with DSi.
Whilst "NGP" has compatibility with digital PSP releases, it won't be compatible with the original physical release, so it won't ever make the 100M list.
PS2 has been hurt by it's removal from PS3, it will be interesting to see if PS4 will be compatible with PS2 or even PS1.
Wii (NGC/Wii) - 84.64
NGC (NGC) - 21.74
DS (GBA/DS) - 144.59 - 32.97 = 111.62 (GBA)
DSi (DS/DSi) - 32.97
GBA (GB/GBC/GBA) - 81.51 - 2.42 = 79.09 (GBY/GBC)
GB Micro (GBA) - 2.42
GBY - 118.69
GBC (GBY/GBC) - ~50
PS3 (PS1/PS3) - 47.9
PS3 BC (PS1/PS2/PS3) - ~6
PS2 (PS1/PS2) - 150
PS1 - 102.49