The obvious choice would be the original arcade machine (if one could find such a thing). But I would say for console ports, the DC version is the way to go (I've owned it and it is fantastic). Haven't played the GC version though I can only imagine that it might be a slight downgrade. And I would stay away from the downloadable ones on XBL and PSN since the music from The Offspring as well as the product placement has been altered (at least I think the product placement has been altered).
I think one thing that should be noted is the horrible controls in the PC version of Crazy Taxi; the game only supports digital controls, even when using an analog gamepad. As you can imagine, this makes the game VERY difficult.