I don't believe there's such a thing as "BEST fighting game" Below are strengths of previous big Street Fighter games and what made them each original.
SF4's greatest strength I feel lies in it's simplicity. To me the best SF4 player will be a player that excels in mind games, control, and a diverse bag of attacking tricks.
Third Strike's originality stems from parry, and it's slow pace. A lot of silly and fun mind games resulted from SF3rd Strike. It's the only game I know of that almost every character will do empty jumps hoping to parry their opponent's anti-air and punish with a devastating combo.
SF2 is the most simple and basic of the Street Fighters, small changes in timing and moves will affect the match, I love it because the strategy is so simple and bare bones. I feel like SF2 is the Street Fighter that really taught me how to play a fighting game without mashing, how to stop this strategy with this etc.
MvC2 is the technical fighters dream, hundred hit combos, reversals, etc. A player that excels in technique, speed, defense, and timing will find a good home here.
You can see the fighting games are all so different to say one is better than the other is really impossible, like comparing apples to oranges. If you look at pro fighting game players you'll see each persons style gives them advantages depending on the game.