Super Street Fighter 2 X- One of the 5 billion different ports of Super Street Fighter 2 Turbo. It's a good version of one of the oldest Street Fighter games, but due to its very high price and being Japan only, it's really only for hardcore collectors.
Street Fighter Alpha 3- One of the most complete games in the series, it has tons of game modes, one of the biggest character rosters, and very fun gameplay that's similar to old school Street Fighter but still feels new and fresh at the same time.
Street Fighter Double Impact- A vastly different game from all the previous entries, it has an almost completely new roster with very few returning old characters. Gameplay is very smooth and the graphics and animation are amazing, but overall you're better off instead getting...
Street Fighter Third Strike- A direct sequel to Double Impact, featuring more characters and more refined gameplay. An all time favorite among hardcore fighting game fans due to its very deep gameplay. Has been played in tournaments for many years.
All 4 are good games, but overall, if you're a more casual fan of fighting games or you're looking for one with a lot of game modes, get Alpha 3. If you're a hardcore fighting game fan looking for a super deep competitive fighting game, get Third Strike.