I've liked all the street fighters up till now, including SF4 and 5. I think 3 was my least favorite since I didn't really like the roster though the gameplay was solid. (And dat animation!)
For my favorites I'd rank it:
Super Street Fighter 2 (the best version of the one that got the craze going)
Street Fighter Alpha 3 (loved all the options and characters though some of the 'isms' were really unbalanced, and that really shows now after the more balanced recent releases)
Street Fighter 5 (this one gets way more hate than is due - they did change it up alot, but the art style is improved from 4, and all the characters truly feel unique with the V-Skill system. The only thing I don't like is how combo heavy it is, but it's still a way better combo system than something like MK)
Street Fighter 4 (basically an updated version of SF2 in many ways, but with an uglyish art style)
Street Fighter 3 (didn't like the roster much or the stages/music, but again gameplay was solid)
From top to bottom there's not alot of difference though! The rankings are not worlds apart or anything - the gameplay is solid in each and every game.
And just for fun, favorite characters from each series (though the thing I enjoy most is random selecting characters and doing matches)-
SSF2 - Cammy, Fei Long, Sagat, E. Honda
SFA3 - Dan (lol), Adon, Rolento, Akuma
SF5 - Sagat, Laura, Menat, Zangief, Vega
SF4 - Dee Jay, Hakan, T. Hawk, Crimson Viper, Juri
SF3 - Twelve, Chun-Li, Elena, Ibuki