Good games library and affordable: Xbox 360. It can be unreliable, but I'm still on my 1st after 2 years.
Expensive, but with great graphics: PS3. It does, however, have a much smaller games library, and they all cost a lot.
Entertaining, especially with a group: Wii. They have a good list of games too, although they are well known to not be "hardcore" -shudder- gaming systems. They are definitely at their best if you play with a group alot.
The first two are probably best for single player.
I recommend the 360, but that's mainly because it's the one I'm most used to. The other 2 have their merits but I'm cheap and like a normal controller over a motion sensor.