a combination of how good they are (usually top tier), their character design and gameplay.
sentinel, duo lon, rolento, vega and nina are my favorite characters because they met all my criteria. i like their design, their gameplay and they are all top tier except maybe rolento but hes up there.
however there are some character designs i dont like too much (not hate) but i just love their gameplay like cvs2 sakura, makoto, st blanka, st fei long, st sim and especially cvs2 kyo
and their are some characters gameplay that i dont find too fun but i use them cuz their character design are tight like yamazaki, strider/doom, guile and charlie.
and top tiers, i never use top tiers just because they are top tiers. i use them on a combination of gameplay or character design. for example, cvs2 bison and cammy. i dont like their gameplay too much but i like their character design. opposite goes to sagat and sa3 yun. i dont like their character design too much but their gameplay is pretty fun and they are strong as fuck.
I usually perfer characters that has off the wall moves with long range and mobility like gen, vega, rolento and rekka based charactrs (duo lon, fei, iori, kyo, etc).