Nezha/Rhino are pretty easy mode, Rhino can get some pretty silly ironskin numbers into the 300-500k with the right build. While Nezha is pretty similar. Nezha gives a huge damage vulnerability to enemies he hits with his ring, while Rhino gives a damage increase to all of his damage. Nezha gives more, but affects enemies (so less) while Rhino buffs himself. Rhino is a bit more microintensive since you need to use his ironclad charge.
Xaku is pretty good but as he only has dodge chance he can end up dying out of nowhere. You have to play a bit more quick for when your shields break but he has defense strip, scaling damage, the ability to kill enemies while not looking at them, and his 4 makes enemies weak to void damage which his scaling damage (stolen guns) do.
Zephyr probably is the easiest/best one (but also a bit boring), she can ignore all damage while grouping all enemies and dealing damage to every single one. While also increasing her crit chance.
Citrine is really fun has she has a unique build that not many frames use, instead of using flow you run equilibrium and synth fiber to infinitely pick up health orbs, which constantly gives her energy. As her 1 makes enemies drop health, it's an infinite loop. She also has 90% DR constantly due to killing to her entire team in affinity range, and she can give a huge critical buff when she stuns enemies and you shoot the crystals sticking out of them.
Revenant is pretty much is unable to die or take damage, and he can kill his thralls (who can be any enemy except eximus with overguard) with a 50% health drain that is unaffected by armor. So he can technically kill enemies to level cap. As long as you keep his mesmer skin up, nothing can harm you.