Real system? I suppose you mean home console, as opposed to handheld devices like the Game Boy or Lynx.

Of the current generation, I'd have to go with the Gamecube. The other two systems are near identical in software terms, with only a few exclusives standing out. The 'Cube has plenty of quality first-party software, which means you won't find it anywhere else. It's also technically the strongest of the three in my opinion. The fact that it has done so poorly is evidence of how screwed up the games industry is at the moment.

Of any generation, I'll go with the SNES or the PSX. Both had a huge range of fantastic software, and provided me with years of gaming enjoyment. I'd be tempted to say that the SNES was better, but I'm well aware that my sense of nostalgia would be a large factor in making that decision. So I'll say a tie between the two.