Depends on if you count demos...
If yes: Brawl, at a best buy. I was Ivysaur fighting against an Ike CPU on what could only be Bridge of Eldin. The Ike CPU didn't really do much except walk up to me, and all I did in return was spam the jab attack. The fight was cut prematurely because my Mom didn't like that I was playing a T-rated game (I was really young at the time).
If no: SEVERAL years later (2011), I'm over at a friends house. I see folks playing as all kinds of weird-looking characters. I hear that Sonic and Pokemon are in it, and that's what catches my interest. iirc, my first match was as Pikachu where I just spammed Neutral-B at a Diddy Kong CPU on a custom stage.