I like having lots of options. Gen is just everything I wanted and more.
Then I subbed Akuma because he’s kinda easy.
Clicked on a random youtube video one day for sf4, it was a rufus vs honda match. The rufus won, and from then on i went and bought the game and had my heart set on the fatman (i secondary honda now to).
in vanilla rose was the first character i learned. Then i got bored with her and tried out Chun li got pissed because i couldn’t do charge moves and then i picked Cammy i loved her fight style and she’s been my main until yesterday. I learned how to do charge moves with my dual-shock and now Chun li is my main.
At the character select screen? 
I specifically don’t look at tier lists because I don’t like them influencing my decision (although I don’t run straight to the top end, I still feel like it might put me off a character I like if they are too low). I play the game for a little while, try every character who’s design I like once and go from there.
SFII: Guile, Balrog
SFAlpha: Guy
SFIV: Balrog, Guy
SFIII: Hugo
MVC2: Doom/Cable/Juggernaut
MVC3: Still torn on 3, I used to play Doom/Sentinel/??? but I don’t play the game at all now, it would definitely be Doom though.
MK: Kabal for sure, he’s about the only character in that entire series I like.
SC: Mitsurugi/Astaroth
Tekken: ??? Never did play enough of that to pick a main.
The downside to doing it this way is that there are character designs I hate and never give a fair chance but do end up finding out later on I like their play style, in SFII this is Dhalsim for me and Rufus/Adon in SFIV, and Bison (Dictator) in any SF game. Love their play styles, hate the characters (cept Gief, he’s boss).
I just pick who I like, sometimes they are pretty high up (Balrog in vanilla SFIV) sometimes they are bottom (Hugo in 3rd Strike).
Hard to say really… When I still had my copy (A new one is on it’s way now), I had Ryu as my main and I was switching between him and Akuma. Akuma had strong attacks but I liked Ryu more because it felt like I had the most control with him.
SSF4 was (pretty much) my first SF game (Except vague memories of Street Fighter 2, Dhalsim, Vega, Honda)
My character I chose when I first got it was Vega because of my memory of him jumping off the wall in that game.
However when my brother proposed I pick a character to be my main, Looked around and Gouken stood out to be badass.
Never looked back.
I’m also quite a RP’er so I love the story and the rivalry between him and Akuma, and I’m a nice guy with high morales so I choose Gouken who is the same.
I’ve been playing since day 1 vanilla and I still haven’t.
I picked sagat first to learn the game but then I moved on to adon because I realized I actually enjoy converting footsies into a pressure game.
I picked Ryu/Ken at first, just like anyone else. And as I grew with the series, I started trying out other characters. Over a year ago, on my 18th birthday, I picked RANDOM and stumbled upon Balrog.
Never once had I noticed him. He’s been my main ever since, I love boxing and Mike Tyson’s hands down my favorite Boxing Champ of all time.
Vega because Street Fighter (1994). I thought he looked badass when I saw that movie as a kid. Ever since then, he was my main.
Picked Ibuki at first, but as someone who never played a SF game before , needless to say she was way WAY too difficult for me at the time. Despised doing links, and thought her having target combos would help me, hahaha how mistaken i was. Anyway picked her because she was cool looking and really like her playstyle. Cody was one of the other characters i liked but thought he was a bit simple, how wrong i was again. Saw someone using Cody once in a match and went absolutely berserk with relentless pressure, didn’t think that was possible with Cody but that inspired me to put some time in him.
Cody is really beginner friendly, and really teaches you some of the fundamentals, especially a good defense, blocking and anti-airing, even a bit of zoning, something i really needed. Also hated links, again something i needed to learn and Cody has a whole lot of them from very easy to very difficult.
Still play Cody, although i fiddle around with Ibuki sometimes but more in training mode. Picked up Cammy along the way because she somehow is very similar with Cody when applying pressure and performing links and i wanted a gtfo move for once haha.
Picked Jin and Steve in SFxT for instance, both hit hard and allow me to press my advantage.
My main is Chun Li. Always been my favorite Street Fighter character but I honestly hate that she’s a charge character. Still love her though lol. I recently just picked up Guy because I like his playstyle. He is that type of in your face character and I like his mixups but you also gotta know when to chill with your pressure game. Another thing I like about Guy is you gotta play smart and bait your opponents into doing stupid shit to punish them. Guy is no easy character to pick up though. Some other reasons I picked up guy is to work on my execution and cuz he’s a ninja who wears sneakers. SWAGG!
Cup size. Obviously it’s hard to gauge because they are cartoons but if they look in the C-DD range they’ll work. If they are a really fun character, I can settle for As or Bs. Like Noel Vermillion. Ibuki is probably wearing a sports bra so its probably pretty likely she’s in the C-D range. She’s still a good pick.
For SSF4 I picked Juri because of nationality (we’re both Korean). Turns out, I liked her fighting style a lot even though I’m not that good with her.
For UMVC3 I chose Taskmaster as my first main because his alternate costume looked way too cool.
I’ve always chosen my mains based on how much fun I have playing them and how flexible they are as far as what they can do. The more variety they have in moves the better for me. I also seem to have a thing for the underdogs, if people tend to ignore a character it only motivates me more to play them.
I am a notorious tester. When i would get a new fighting game, i would spend a few weeks in training/arcade modes trying out EVERYONE. I would also try to stay away from internet forums (once they were a thing that existed) so i wouldn’t be swayed. So, i got pretty self-aware on what i liked and what would work for me. In SF games, i tended to go for either charge characters, or quick strike characters. In KoF, i tended to go for the more balanced quick/strong characters.
Examples:
SF2: Guile and Blanka
SF2turbo: Guile, Blanka and Balrog
SSF2turbo: Cammy, Dee Jay, Blanka
SF Alpha 3: Karin, Cammy, Adon, E Honda, Dee Jay
CvS2: Blanka, Rock, Terry, Joe, Iori
Mark of the Garou: Freeman, K.Butt, Rock
KoF '98: Robert, Blue Mary, King, Joe… and Lucky Glauber (true story)
KoF 2002: K, Iori, Joe, Robert, Blue Mary
KoF XI: Ash, Iori, Blue Mary, B. Jannet
I’m just getting back into things and have yet to buy any current gen fighting games… but i look forward to getting into KoF XIII, SF4 and SxT!
I personally go with the character design, but later on i go with noobie characters because I’m not good at fighting games 
Akuma is a terrible choice for a new player. I’d recommend Ryu or Guile personally.
The way that I choose a main is based on their style. I dont see the characters as anything other than fighting styles. Chun Li is all about footsies and zoning, so I play her. Ryu is all about (in order) zoning, big damage and footsies So I play him. Akuma has great footsies, really good rush down ability, and decent zoning ability. so I like him too as hes probably the most dynamic character for me.
Characters that have good footsies are the characters that I lean to, the second priority is zoning