Hooo boy.
Okay. So. Best is not the same as easiest. Some characters are easy, but not actually very good because they lag behind once the opponent is executing at a similar level. Some characters are really hard, but once you master some of their tricks become extremely strong. Or maybe you’ve got a character with strong basic tools that can also go a long way. (Lookin’ at you, Ryu.) And “what makes a character good” is an extremely complicated discussion.
On the one hand, you have the school of “matchups” where you examine how a given character does against the rest of the cast - maybe Zangief does well against almost everyone, but gets totally blown up by Dhalsim. Or maybe E.Honda is actually kinda bad, but can basically downback and react to anything Cammy does. So then you kinda average everything together and that tells you how ‘good’ the character is.
On the other hand, you can get more abstract and start discussing things like space control - Ryu controls the ground game pretty thoroughly compared to Ken due to faster recovery on his fireball, but can suffer against characters who have a way to evade that attack. Dhalsim doesn’t suffer from that limitation because he has normals that can’t be beaten by ‘anti-projectile’ moves (moves that “Go through fireballs”) and that control different parts of the screen (Standing Roundhouse can snipe people out of a jump almost at fullscreen range.)
Anyway, unless you restrict yourself to discussion of a single game (Say, Super Street Fighter 2 Turbo) you can’t really analyze this stuff at anything more than an elementary level because things like recovery time changed between versions, so a character who was good in one game could be merely indifferent in another.
That said, if you want to talk about that game, you can start by looking at something like this.; It’s sortof the collective interpretation of a whole lot of people who have been playing the game for a long time. Popular wisdom, therefore holds that Vega and Dhalsim are very good, Chun-li and Balrog are well above most of the competition, and Zangief and T.Hawk kinda suck. It’ll even tell you “why” in the sense of who people think different characters lose to, but it won’t tell you why they lose to those people. It’s all just opinion, but it’s usually educated opinion. Usually.
Generally though, characters who are strong are the ones who are good at controlling space; Note that that doesn’t necessarily mean the characters with long range attacks, though it often does. This is, essentially, the basis of the idea of “priority” (which does not really exist); Who has moves that make it difficult/risky for their opponent to be at a specific location. Zangief controls space very well close up, but it’s difficult for him to get through or close to many characters. Ryu doesn’t control the super close range space as well, since he lacks the threat of the command grab, but his fireball threatens the opponent even at full screen, and he has several strong options for addressing people who jump at him, or who want to dance around about a character length away. etc.
