For me it's more than combos I can instantly pull off. I pick 3 characters for competitive play and at least two for fun.
For competitive characters I choose based on certain criteria.
I'm a rushdown and setup player so I look for the following in my mains:
1. They need good pokes, good advancing mid, fast starter and a decent opener either 50/50s or plus on block or moves with pushback stagger or moves with different number of hits.
For my setup characters I need at least good D4, keep away strings, something to get the setup tool out safely like push back, cancelable specials that have set play like Cyrax bombs. Kabal is a good example of a blend between rushdown and mid range set play.
And I usually get one character who is polar opposite of my mains like a zoning character with mainly keepout tools like Kollector, Cetrion, Jade.
I haven't found my main yet but my favorite is:
Kotal Kahn
Sonya
As possible rushdown mains and
Cetrion
Jade
As my possible space control mains but these characters feel like they are missing something?
Kotal can't get out his Sun Ray, Ground Slam or Totems safley. Otherwise he would be my absolute main and if he had Kahn-cut asbase move I'd be in love with him
But yes I have to also like the look of the characters too.
My best advice for those struggling is to find one character they like aesthetically then find one with the tools you need, I'm playing characters I never thought I would in 11.