You guys all have such good taste.
But I'm going to have to go with Spyro the Dragon, I could play it over and over for countless hours. And back when I was still a youngen I would spend literally all day imagining scenarios in Gnasty's loot after you got 100%
Next up I would say Digimon world 1, I never did complete the chart and collected all the different species, but hey I had fun trying.
When I got my first Ultimate, I was so so so excited, Pheonixmon if I remember correctly. Finished the storyline with that one.
And 3rd I think probably the Crash Bandicoot trilogy. I loved them, I just did.
Also Crash Bash was a very good multiplayer game, I would play for hours with my mates, only two player and swap every time you lose because we couldn't connect 4 controllers unfortunately.