This is actually a really hard question to answer. For nostalgia, it's Sonic Adventure. For online, it's PSO, AFO or Worms. For a casual playthrough on a rainy day, it's Crazy Taxi or Powerstone. Then there's Soul Calibur, Jet Set Radio...ah crap forgot about Rez and Ooga Booga....look, just don't make me choose OK?
PSO - Crimez (Lv136 FoNewm) / Revlis (Lv20 RaCast)
Anything else - dcdingoYouTube Link