surely SFIV, ... given the support and maintenance that Capcom has renewed all these years (since 2008) in the title, no doubt it became one of the most technical, deep and balanced game in the entire series (and perhaps between all the current fighting games general) .. at every big event brings pleasant surprises, and it is still able to surprise with the arrival of Ultra-SFIV ... there is a good amount of characters with great variety, is very well polished visually, fantastic soundtrack, all according to be the best game of the decade.
but in my personal rank (still keeping SFIV in the top), not counting the "Vs. Series", there we are:
Super Street Fighter IV Arcade Edition (currently) > Street Fighter III Third Strike > Street Fighter Alpha 2 > Hyper Street Fighter II > the rest of franchise.