I would personally list;
1. NES - Revived the American market after the crash, also the console to bring Japanese console dominance to the scene, something which still isn't broken.
2. Gameboy - Took handheld gaming mainstream. Simply can't overstate the influence of games such as Tetris and Pokemon.
3/4. Wii/DS - This one is largely based on observation of current events so it may turn out to be wrong, but by the looks of it the Wii is going to lead the largest expansion of the market in years simply by making it accesible to consumers who have largely been ignored ever since the PS1-era (or forever in the case of the 40+ age group). The DS is also leading market expansion into these groups (though in handhelds the younger crowd were never as forgotten as on the home-consoles) and bringing in the golden oldies like never before.
5. PS1 - Led a large change in the market towards a more 'mature' system, increased the emphasis on the 15+ age group rather than either children or all inclusive games that had been largely dominant before this era. Also brought Sony into the market which changed the industry quite a lot.
6. Atari 260. Made gaming mainstream in America, killed gaming in America. Would rank far more highly in the ranks of importance if it didn't undo itself.
7/8. Xbox/Genesis. Introduced Microsoft and Sega to the market.
Unranked consoles + reasons.
PS2/SNES/PS3/X360/N64/GC/Sega consoles - Essentially continuations on existing themes
