Uploading your full roster would help more since some operators are able to function well outside their actual class.
Take Siege for example. Her first skill is the standard Vanguard skill, get DP whenever the SP gauge fails up.
Her second let’s her heavy deal AoE damage, making her a pseudo AoE Guard, better even since it hits all enemies in range regardless of her Block count. This only gives a few DP in exchange, unlike other Vanguard S2.
Her S3 turns her into a straight up Guard, dealing high single-target damage with a chance to stun on each attack.
Most Vanguards are retreated once they help generate enough DP for you to deploy your full team and a Guard or Defender in their place, but Siege is strong and tanky enough to remain on the field the entire fight.
Ifrit has a unique range, if you’ve seen it, as she fires in a straight line, hitting all enemies in her range unlike the usual AoE Caster. This makes it hard to deploy her in some stages, but devastating when she has the right firing lane.
As a bonus, her Talent reduces the RES for enemies in her range, helping out her fellow Casters and Arts damage dealers, plus she reduces the DEF of enemies temporarily when they are hit with her S2.
For the Medics, both Shining and Nightingale are pretty good too. Nightingale is an AoE Medic who provides a RES buff to her allies, and she has a unique ability where you can deploy phantoms to draw fire from ranged enemy units, useful for those maps where you dont want your operators being hit by hard hitting enemy Snipers or Casters.
Shining, opposed to Nightingale, is an ST Medic who l provides a DEF buff instead.
Both will provide some added flexibility and utility to your team.
So the choice, in terms of function, depends on who else you have on hand but all four will open up new options for you. If you can’t decide, just do what a lot of us do. Pick your waifu.