It shuffles between Tales of Symphonia and either a 3D Mario game or a Zelda game, Mario always seems to surpass itself for me with new entries and Zelda finally changed from Wind Waker to Breath of the Wild in the top, Odyssey is looking fantastic as well, so it may be a tough call whenever that comes out.

Tetris. GameBoy. Since 1989.

The original is great as well, maybe in my all time top ten. I tend to play through the first two about once a year. Never get tired of them, no matter how many times I complete them. I do a run through of 3 every now and then too, although I find that a bit more of a chore than its predecessors.

Mine is unchanged since 1993, Betrayal at Krondor is still the best game I ever played, all things considered.

It has changed quite a few times, actually.

First it was Cardinal Syn (ps1). I loved that game to pieces.

Then it went back to Super Mario Bros 3. It remained my favourite game until I played Heroes of Might and Magic 3.
I thought nothing could beat it. Then it was replaced by King's Bounty: Armoured Princess.

Journey was an amazing experience but I could never see it as my absolute favourite of all time.

A few years ago I played Beyond: Two Souls. Half-way through the game, I just knew I found my "perfect" game. Words can't express how much I liked it. I don't see it being replaced by anything else, but based on how many time I've changed my mind it's quite likely.

As others have lamented, a strong combination of your number 1 for the longest time, how it has lasted throughout the years, how you have felt about it when you have played it determines your numero uno game and probably sets it in stone for at least a great fraction of your life if not all of it.

LttP was my first favorite game of all time before I was a FF fan, but then after getting into rpgs through SMRPG my tastes changed a little and then I played through FF7, then FF9. Everything changed and my mind opened up and made DMT trips more interesting.

My #1 game has been Starcraft since 1998, so not often. Before that it was Super Mario Bros. 3

It is my number one aswell. Nothing can beat it. 

Nothing has yet to beat Spyro Year of the Dragon for me.

I can't say that I have one specific #1 game but I have 5 favourite games and this doesn't change for years. I doubt that any game can ever beat something on that list. So I think it will remain unchanged