I think those saying the fighters in Mereen were common slaves is a pretty bad misconception. Mereen houses the best fighting pits in the world and those fighters were taking part in the greatest even that happens there annually. They were all incredibly accomplished fighters. The spearman, waterdancer and dornishman were all accomplished killers.
This is a tough fight to call in my opinion. They're pretty similar in fighting style. Neither is particular graceful. Their fights are rough and dirty. In fact these two folks might be the best swordsman left alive on the show.
Jorah has fought the best pit fighters in the world, beat a few stonemen to death and has mastered the art of one hit knocking out other fighters. Jon killed a white walker in single combat. An impressive feat. Although he had to use Jorah's ancestral sword to do it.
Isn't Ultimate Colossus one of the strongest beings in that universe? He resisted an amped magneto and laid a beating on him! Heck. I want to say the only thing that actually injured him had been adamantium and he ended up winning that fight anyway.