Topic: Which ONE Game have you bought the most times?

Probably Puyo Puyo 2
I got it physical on Super Famicom twice (One with Box + Manual and one without), Mega Drive, Game Boy, Sega Saturn, Wonderswan and 3DS (included in Sega 3D Classic Collection).

I also have digital Versions on Wii, the Sega Game Gear Version on my japanese 3DS and the Sega Ages Version on Nintendo Switch.

A few other games I bought multiple times is

Streets of Rage 2. Genesis, Wii VC, Android, Switch, not to mention the Genesis compilation discs just for SoR 2 on various consoles.

Borderlands 2. Xbox 360, PS4, Xbox One

Diablo III, PC, Xbox 360, PS4, Nintendo Switch.

Diablo 3 - PC, PS3, PS4, Switch

I think that would be Metal Gear Solid 3:

The original one, on PS2. Then again, in the Metal Gear Solid HD Collection for X360. Then it was the Metal Gear Solid: Legacy Collection on PS3. And, finally, on the 3DS.

I wanted to buy the PS Vita version of the HD Collection even I did not have this little console.

@NinChocolate Similarly, I plan to get Sonic Origins just to have Sonic CD on Switch. Haven't rushed to get it since I can already play most of the games on it. Pretty sure whatever device I get next, if Sonic 1 pops up, I'll get it there too, lol.

Sonic 1, the one game that somehow can be played on almost every device released after 2000.