I haven't really got one game that I value above all else, rather, a bunch of games that I rank above all else:
Star Wars Battlefront 2 (the memories of playing this game with my little brother are some of the best)
Super Mario Galaxy 1 & 2 (simply outstanding and breathtaking, the greatest Mario games ever in my opinion)
Sonic 3 & Knuckles (the two best Sonic games of all, the series has never managed to better this instalment)
Lylat Wars (Exciting space shooting action will never stop being awesome, like this game!)
Portal (it might be short, but for what you get it really is the perfect length to me)
Pokemon Platinum (It's hard to explain, but to me this is simply the perfect Pokemon game)
Batman Arkham City (again, it's short, but I just love me some Batman and this is one of the best superhero games ever (alongside Spider-Man 2))
Skyrim (I've only ever played Oblivion and this, and I find this one to be much easier to get in to)
The Legend of Zelda: Ocarina of Time/Twilight Princess/Skyward Sword (JUST PICK ONE OF THOSE, I CAN'T DECIDE!)
Mario Kart DS (Only here because it's the fist MK I ever played, but the series just keeps improving with each game)
WarioWare: Minigame Mania (The funniest game ever, I've invested hours into it, WW:Touched is a good sequel though)
Super Smash Bros. Brawl (I know of all the complaints that competitive Smash players have, but no other game is such a celebration of Nintendo and it's still a very well made and fun game with countless amounts of unlockable content)
The Simpsons Hit and Run (this has so much thought and effort put into it, one of my favourite childhood games)
No particular order by the way.
The game that I consider to be the greatest ever made, Super Mario Bros., is not one of my favourite games ever, there is a difference.