Logically one would pick their favourite game, because that's the one you like the most. Why would you settle for anything less? For me that would be Zelda: Ocarina of Time.
There is a caveat though, if it's a game with a definitive start and end, like any story-based game, will it stay your favourite thing if it's the only thing you'll play over and over for the rest of eternity. Then, I'd say, maybe the best thing be to pick an endless game, or a sandbox. For me the best things in that area are city-building (or just 'building' games, doesn't necessarily have to be a "city") games, and my favourite of all there is RollerCoaster Tycoon 2.
I'd probably pick RollerCoaster Tycoon 2.







