Depends on how you define best. Best gameplay? Graphics? Something else entirely?
For me, this could be the Elder Scrolls IV: Oblivion. It's the one I have the most good memories of playing when I was younger, but you can't really say it's good compared to the ones we have today, like Skyrim, or Dragon Age: Inquisition, or whatever else people play. Then there's Mount & Blade: Warband, which is an awesome game with amazing gameplay, but lacking in graphics, and with the expansion Mount & Blade Warband: Viking Conquest coming out tomorrow, the 11th of December, I'm going to have to be careful not to forget all my school assignments...
Must not forget Minesweeper, with its great gameplay and amazing graphics. Yeah, it's definitely the best game. Definitely.
Though seriously, I've spent way too much time playing that game, and I'm only just under 100 seconds. 93 seconds on expert level is the best I've done, if I remember correctly.