When it comes down to greatest for me, I think of a combination of groundbreaking systems combined with vast libraries and exciting, fresh titles. From my own personal experience I'd say:
1. NES - memories combined with game playability and library. How many hours did you spend playing Zelda or Punch Out and countless others? What this system did for gaming history can probably never be duplicated, as it brought gaming into the mainstream and towards the forefront of entertainment for a generation, while creating a marketplace for the systems of today and tomorrow.
2. PS/PS2 - incredible library, and the system created a whole new level of gaming experience and capability (which has obviously been surpassed and will continue to be, but at the time was incredible).
3. Gameboy - Creating a portable system was a groundbreaking idea, and although the games were simple, playing Soccer, Paperboy and Megaman in the car on those long summer vacations with the rents was such a fantasic option for kids like myself.
You could make an argument for every other system having its moments. For example, I loved the fact that Genesis's games were so much more vivid and graphic than the NES, which was groundbreaking for me at the time. But just about every system has its roots in these, so that's why I'd rate them in greatness this way.
Here's hoping that I feel the PS3 is the best five years from now, so I get back some of that expensive investment.