In all seriousness, Ecco the Dolphin for the Sega Megadrive/Genesis.
I spent so many hours as a kid swimming around, exploring, getting lost, trying to stay alive and generally just having fun. Not to mention the music is so... soothing. I can see why people dislike it, because it's incredibly fustrating and let's face it, it's a dolphin.... but for some reason I had a really awesome experience with this game, so I love it :3
Also, Another Code: Another Memory (Trace Memory in the US) for the DS and Another Code: R for the Wii.
I mean, I know they could be slow at times, and half the time it wasn't clear what you're supposed to do... but no other game managed to make me weep the way these two did. And I'm not a soppy person- they really are touching. Also, I liked the way it was up to you to put the pieces together... heck, in Another Memory there were several offshoots from the main plot that you could complete, and only then did you learn the full story- but it was up to you to figure out what went with what. And I really enjoyed that.
Mini Ninjas as well (for just about every console today)... I had the Xbox 360 version. People criticise its wasted potential, but it's been a long time since I had that much fun messing around in an overworld that didn't belong to a Zelda game. Some of the humour was rather childish too... like Windy Pants (one of the bosses... I'll leave your imagination to figure out what his main attack was), but sometimes games need a break from being totally serious. The game doesn't take itself seriously, and that's what I enjoyed about it. XD
Aaand, I guess I should mention TAoL too... there's a feeling of responsibility and freedom in that game I haven't found in any others, not even other Zelda games. I felt bad when I got a Game Over, like actually, a really terrible person, because it makes it clear you just doomed Hyrule every time you do. It's up to you to make sure you stay alive, because there aren't gonna be fifty million fairies to constantly revive you when you mess up... it really kind of hands you the reigns and that's just awesome. Also, the fighting system is top-notch, and I'd better shut up now or I'd go on all day about this XDD