NES.
The Nintendo introduced the mainstream population to consoles. This is especially important when the Atari had left a bad taste in peoples' mouths. It also was the system with the largest collection of games (granted not all of them good) and the longest shelf life ('85-'92) for a system that still had cartridges. Every other system released after the NES had a competing system (SNES vs. Genesis, PS vs. n64 vs saturn ect..) and never had as broad of an appeal that the NES did.