PS2 by far. It has the longest life of any system to date, not to mention some of the best games (GTA, God of War, Ratchet and Clank, Jak, etc). It's still a good system by today's standards.
On the other hand, the Wii is easy to hack to play backups. You could probably buy the system, a SD card, a WM+, and a nunchuck for a decent price and be able to play practically any game for the GC or Wii on it for free. Try doing that with a 360 or PS3.