These are the best characters, they will work in any run to the same value, no matter what, and can only become better by being upgraded during the run itself. They offer very high value to the team or themselves, which makes them even with a shit build, insanely strong.
Spotter
Spotter is currently insane with the value he brings to a team, especially in deeper depths where enemies both become tankier and hit harder. His main value comes from his Crit Dart, and with that, his innate synergy with all crit augments. He can single-handedly give your team double damage vs bosses and elite enemies, which really helps when you are on a timer.
The Radar he gives is fine, it's not game-changing, but it definitely helps when finding what objectives are on the map.
His Ult is very good, especially on bosses and elevator sequences when a ton of enemies come, and you don't have time to find a restock machine.
Because of how his abilities work, he is generally just a monster with the correct upgrades, workbench upgrades, bio enhancers, and eventually artifacts.
Falconer
Falconer is just an all-arounder that can do anything, but is still by far the best as a DPS. She has very good elemental damage because of her Thunder Rod and Shock Drone, both giving her insane shock potential.
This becomes especially cool when you get upgrades that synergize with shock, because she is the only character that can reliably use that without having a specific gun or upgrade for that gun.
Her Shock drone also does great damage and becomes very strong once you have a few workbench upgrades on it.
But in my opinion, her most broken ability is by far her Ultimate, Remove Drone Revive. This allows you to revive a companion from a distance, which cannot be interrupted. This is insanely high value, especially in deeper depths, during waves or boss fights, when getting to allies to revive them without being pummeled by enemies is very hard.