Any tbh, its not the gear that makes you strong its the food.
Flit spear scales ok into late game because of its turbo low stamina cost, thou Mistlands might be a streach... Plains/Swamp i often do exactly that in my games.

Everything else will depend on food, if i can have mistlands food, starting area gear is enough to fight "normally" in plains and anything below...

But to answer the question... still Swamps probably because you can play normally with this gear there (using meadows best hp food). Plains would be easy to traverse but teadious to actually do stuff. Same with mistlads but worse since seekers dont have 10hp.

Also weird Mountain is not on the list... you can traverse terrain with campfires and caves are plenty doable with bronze shield/flit spear already, i usually skip all metal armor and go straight troll->fenris->endgame.