Best != perfect, Perfect != favourite, Favourite != best.
Best implies superiority over all others, you can definitely say who is the best sprinter by seeing their times, you can say who is best at high jump by looking at how high they jump.
What is best in a video game? Enjoyment? Emotional attachment? Innovation? Most sold?
All of those things could be what you consider best, it could be a combination, who decides the weighting?
Best in video games, is subjective, not subjective enough that you can run around telling people big rigs is the best game ever, but enough that there is no clear cut best game.
When it comes down to it, my own opinion (and it is *just* an opinion) best is a game which you believe was the greatest masterpiece. For example for me that would be half-life 2, as it combined such great narrative pacing, fantastic art style, music, technival innovation etc.
However ask me my favourite game, and that would go to diablo 2, for the hundreds of hours of enjoyment it brought, and the emotional attachment I have with the music.
Do I regard diablo 2 as the best game ever? Nope, there are too many flaws for me to call it "best". But it is my favourite.
Portal could very well be called best by many people, I don't believe it is personally, but I could understand it being named so. It is around the level of games which are able to be called the best, up there with ocarina of time, half-life 2, system shock etc.
So No, not the best game ever, but imo worthy of it being named so.