well, inaros is the king of not dying. you have to actively try to get killed when playing him.
rhino is a tanky boi but gets fucked by nullifiers. new nezha is essentially rhino but faster and gayer.
nidus is tanky but takes a while to ramp up since you need mutation stacks to trigger his passive, so he's not really useful on shorter missions or vs. enemies he can't stack effectively against.
chroma got nerfed hard, but he still should be pretty tanky. you just have to choose fire or ice for your elemental color.
limbo is untouchable as long as you're in the rift, but be prepared to have teammates utterly despise you if you want to play him. he's squishy outside of the rift though, so you gotta manage it properly.
nova and mesa aren't conventional tanks but can get 90-95% damage reduction with their 1 and 3 with enough power strength, respectively.
wukong can't die with his 2 active, but it's literally his only useful skill and he's boring as shit to play.
gara gets 90% DR with her 2 and also has infinitely scaling damage on it when you combo it with her 4 and 1. probably the best scaling tank frame right now.
valkyr is unkillable with her 4, but it's a pretty shit exalted weapon outside of that. but she's also got a metric fuckton of base armor.
baruuk becomes unkillable with his 1, but it only works from certain angles, you have to mod for range to max out its effectiveness, AND it only works when you're not attacking anything, which is stupid. also he's a pain in the ass to farm.