Probably Super Smash Bros. Melee, I played it every day all afternoon with a friend, we got really good at it and eventually started organising tournaments. We were pretty young so we came up with stupid ideas like the other contestants having to fight and then we would be the ''final bosses'' you had to defeat to win the tournament. Ofcourse after months of nonstop ''training'' no one stood a chance

It never really got boring and the only reason we quit was cause the Wii and Super Smash Bros. Brawl released, but it never gave us the same feel so we just quit the whole thing. Sort of a shame.
I would pick Ocarina of Time, but during that time I was too young and scared to leave Kokiri Forest so I basically let an older friend of mine finish the entire game for me in which I did nothing else but have my own little roleplay sessions in Kokiri forest, pretending I was ''sword training''.