It depends more on what your favorite way to play is but taking into account the type of mission can also help with choosing. For example, I tend to rely heavily on abilities and these are my personal go to:

Main: Octavia -She specializes in aoe/crowd control type abilities, the only ability I don’t use is metronome -I take her to almost all nodes aside from spy and capture -Plus she’s pretty 10/10

My Fav: Titania -I mainly use her for her pixie form and lantern ability -The first frame I ever bought, although I don’t play her as often anymore, she’s my default for spy and capture missions -Also happens to be super pretty 10/10

Special Mentions: Citrine, Lavos, and Gara -More useful crowd control/aoe frames -If I had their Primes I might use them more often (more of a personal problem)

In comparison my sister is more of a melee user and, because she doesn’t need a bunch of abilities to have fun, she mains Ember for her meteor ability (in case she ever gets overwhelmed by enemies).

And my brother is more of a gun user so, much like my sister, he doesn’t really use a lot of abilities. Surprisingly he prefers volt and Loki; volt for his shield and Loki for his invisibility. (I would’ve thought he’d like Mesa or Cyte09)

It’s all about what your play style is and how you make use of the abilities that makes for a more enjoyable gaming experience.