JJ you bring up an interesting point - but here is the answer:
The NES. I've never seen reliable online figures for the NES, but Steven Kent's book - The Ultimate History of Videogames holds that the NES - sold well enough over 10 years at USA retail - 85',86',87',88',89',90',91',92',93',94', to remain on store shelves as long as PS1 did (95'-04'). From 86'-89' NES sold 5 million+ units per year in the USA according to figures in Kent's book before slowing down a bit as Genesis released. It continued to decline - but very, very slowly - even after the release of the SNES in 1991 and 1992. Put it this way: in 1994, in it's 10th year, and with well over 30 million units sold (not shipped) in the USA, the NES sold more than Dreamcast did in year 4 (2002, when the market was bigger and Dreamcast was cheaper than NES - adjusted for inflation - in it's year four), and more than the Sega Saturn did in it's launch year. Again, I've never seen complete NES figures worldwide, but it sure as hell didn't decline after year one. In the USA, to reach that 30 million level, I imagine it went like:
85' ~ <1 million (it wasn't launched nationally at once)
86'/87/88'/89' - 5 million-6 million
90' - 3-4 million
91'-92' - 2-3 million
93' - 1-2 million
94' - 300k-1.3 million
95' - 100k-400k
96' - 0-100k
97' - <50k
My theory is NES outsold the SNES worldwide in 1991 when SNES was available worldwide (remember - Genesis didn't do too well in Japan - and back then Japan was still a bigger market than Europe. NES continued to see decent games released as late as 1994, and was still sold in the 2000s in Japan) and was probably neck and neck in 1992 with the NES. The SNES didn't really take off until 1993 when it was a) cheaper, b) Genesis became old/dated in the West, and c) software studios in Japan needed a new, big base to support (Genesis sold less in Japan than a typical Final Fantasy game sells). At the same time, PS2 immediately (2001) began outselling it's predecessor.
Basically, the NES was the best selling console in Japan for a 6-10 years (83'-88' at least, and outsold next gen consoles in the west until the SNES finally got people to stop buying NES games in 1993) - against Sega, NEC, and others, the best selling console in the west - even against the Genesis (initiailly), and SNES (initially). When a console company gets 70% of the market, it takes a fundamental industry shift - in at least 2/3 of the three markets - to lose momentum completely.