Calling Twilight Struggle the best boardgame on the basis of its BGG ranking is quite a stretch. BGG rankings are not to be taken as absolute statements of a game's quality or popularity. Just take into account that most people won't necessarily go back to rerate a game after a few years have passed. Also, a game which would never even be considered for play except by the enthusiasts (like Twilight Struggle...) will have its score inflated. A game like Dominion, which was much more popular and had a significantly higher impact on the industry sits lower in the rankings because its mass appeal meant it was exposed to more people who weren't enthousiasts of its genre, and its huge popularity led to people playing derivatives of Dominion before playing it, also negatively affecting its score.
The reality is that Twilight Struggle is a heavy, sort of bland game which appeals to a very limited crowd of enthousiasts. It also has had little impact on the industry as few games copied its system. I find it difficult to qualify it as the "best" game. I think Dominion and Pandemic are much better candidates for this title, at least in recent years, based on their impact on the industry.
Certainly, the industry is currently moving away from heavy games like Twilight Struggle.