I don't know how to define the best game ever but I'm gonna list the few games that just completely captivated me for their time and I was blown away.
Resident Evil
WWF Warzone
Uncharted: Drake's Fortune
Resident Evil I didn't even play at first. I stayed up all night on christmas break watching my older brother play the game and I was just completely captivated by the game and even more so when I finally played it myself. I never was so afraid of dying in a game before in my life, and I never have been ever since. Resident Evil gave me a feeling of survival that no other game will ever give me ever again(RE2 & 3 come close). The only other thing that can come close is when the score is tied 3-3 on search and destroy and you hear the "Comrade, get it done, you're the only one left." I love that pressure.
WWF Warzone, I was in middle school and just got really big into wrestling from my friends talking about it. I finally got this game and it was a combination of me being consumed by the wrestling world and the huge leap the game took compared to the old wrestling games. I was blown away. I didn't have cable so I didnt get to watch Raw, just the saturday night shotgun program that came on at 1am on local tv, so this game was one of my only ways into the wrestling world which I had little access to. This was also before I had/knew of the internet.
Uncharted: Drake's Fortune is probably the best game I have ever played. It did everything I ever wanted in a video game. It gave me great story, great character development, great pacing, great atmosphere, great graphics, great action, a sense of realism and fun gameplay to boot. I still think it is light years ahead of Uncharted 2 because it gave that epic feeling without having to result to extreme(even for video games) over the top action every 5 minutes.