Street Fighter IV (PS3/360/Arcade, 2009)
Super Street Fighter IV (2010)
Super Street Fighter IV: Arcade Edition (2011)
Ultra Street Fighter IV (2014)
Stretching back to the 16-bit days of Street Fighter II and its various iterations - or whichever one that allowed the player to use the four boss characters Balrag, Sagat, Bison, and Vega (yikes, who can keep up with all the SFII releases?) - my main character of choice has always been Vega. Clearly something is wrong with me - Vega, while quick and agile, has a terrible defense and an offense that actually can be downgraded halfway into a match (when he loses his claw after so many hits). There's a reason he's ranked so low among the character tournament tiers - he's deeply flawed.
See, I never really got to practice much with older SF games. Sure I've owned them over the years, but playing with a limited number of players can only take you so far, and as for arcade mode... well, you get to the point where you cruise through it or hit a wall, feeling like the game has become insurmountably cheap. Street Fighter IV changed that, for me personally, because it had a mostly-smooth online component that (during its peak years, at least) offered an endless series of matches against a vast range of player skill. The same could be said for any fighting game, really, but playing in this way was a big step in forcing me to think about new strategies other than wall-jump-spamming and other cheap tricks. It also revealed, on the rare occasion I would face another player using Vega, just how downright annoying the character can be. Naturally this increased my love for the character. Vega has a weak close-range defense, but he's a master at agile keep-away, and you don't have to be skilled to drive an opponent nuts with that tactic. Needless to say I got my ass kicked most of the time - but I still enjoyed it, weak Vega be damned.
Which isn't to say that a skilled Vega player couldn't make quick work of any challenger. Alas I am not a skilled Street Fighter player. That's the beauty of the series, though - it doesn't really matter how skilled you are, as long you're not terrible; the series is so immediately accessible and simultaneously deep that even scrubs like me can enjoy it on a regular basis. Street Fighter IV was a comeback for the series. Just because SFIII's bold changes - and massive roster overhaul - put the series out of the public gaming consciousness for a few years, it wasn't like Capcom was resting on its laurels. In the years between SFII and SFIV, Capcom released the excellent prequel series, SF Alpha, which culminated in the masterpiece Alpha 3; they experimented in the 3D realm with games like Rival Schools and Power Stone; they pushed new and unusual 2D experiences like Darkstalkers and Jojo's Bizarre Adventure; they practically created a new sub-genre of fighting with their over-the-top Marvel fighters. But when SFIII left most players - except the hardcore fighting fans who could appreciate the deeper mechanics and new roster - feeling cold, Street Fighter IV is the game that brought the series back to the forefront of the genre; the defining fighting game of its generation.
All screenshots are my own. SFIV screenshots taken from the PS4 port of Ultra Street Fighter IV. Box art from GameFAQs.