Night Witch

Who?

Mountain

Mountain is not a great fighter. I've don't remember him defeating anyone of importance except Oberyn and that too because of Oberyn's mistake. Oberyn could have clearly killed him easily. He lost to Loras Tyrell in the Tourny for the Hand but you could say it was a joust and not a real fight. But still, Jousting requires many of the skills that fighting involves. He relies more on brute strength and size advantage.

So, by the end of season 7, would Jon be on the same level of fighters as Jaime, Barristan Selmy, Clegane, and the Mountain? If not 1v1, could he be compared to those fighters in a skirmish type setting out in the battlefield during a war?

I do think Jon is one of the best swordsmen in Westeros because he's been constantly fighting in near death situations while others have blunt their skills over time.

I don't think he's at the level of Barristan Selmy or Jaimie (when he had both hands) yet. I think he can defeat the Hound but with difficulty. I think if he fought in case of Jon vs. Jorah it would be 55 - 45 in Jon's favor. Jorah is old now. (In his youth Jorah defeated Jaime in jousting and won the tournament at Lannisport. Again, you could reasonably argue it was not a real fight, but it does show he has great skill. He also defeated a Dothraki Bloodrider)

I am also not sure he can defeat Oberyn. I think he is equal to or somewhat better than Eddard now. Remember Eddard was able to hold his own against Jaime in season 1 until someone put a spear through his leg. Eddard also put up a decent show against Arthur Dayne - he won by cheating though.