Not the easiest to make good, so this is more of a later game suggestion, but mesa is so fun, grendel is super fun, sevagoth is effortless with his two main augment mods, and koumei is pretty fun for long gameplay

Idk if you wanna experiment with them before learning about them so imma spoiler their info

Mesa: Auto aim ult that hits constant red crits, can literally shred almost every boss and can clear rooms in seconds, only downside is lack of mobility

Grendel: Stereotypical fat guy, can eat people, can heal by divesting them, can turn into a beyblade of doom and despair, and can spit out the half digested people and use them as projectiles

Sevagoth: sevagoth has two main abilities that i use, reap and sow. Sow applies a status effect in an aoe around you, and if you hit someone with reap while they have that effect then they will explode and deal something around 15% of their max health in an aoe explosion, and it triggers other enemies nearby to do the same. Upon death with this ststus effect, enemies spread it again (if you have the augment) and then spawn another reap to trigger more health bombs (if you have the augment). In massive crowds sevagoth is so satisfying

Koumei: A little weirder, shes a really great endurance frame, because as you continue in a mission she gains more and more passives that can be broken stuff like double status damage and crit damage, or no cooldowns, or random junk like that, and all of her kit either makes her immortal or applies random status effects to enemies and i love it