Not sure if we count the Mountain, but he was obviously very formidable as well. And Oberyn would be up there as well if he were alive, because he beat the Mountain in single combat.
Loras is another guy who would probably be top five if he hadn't just died at the end of last season.
Of the people still alive, I'd think Jon Snow would have to be in the conversation on the show at least. We've seen him win some pretty tough fights with some pretty formidable foes. Book Jon doesn't really have that, but it seems like we're talking the show here.
Daario Naharis should probably be on the list as well. We've mostly only seen him kick ass when he's needed to.
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If we start talking books, I think it'd be:
Brienne
Loras
The Hound
Mance
Barristan
Don't really know who would be the fifth.* I put Mance on there because he pretty effortlessly beat the shit out of Jon Snow in the practice yard, and he had to beat a lot of great wildling warriors to become King in the first place. He's also, as far as we know, still alive in the books. The Hound, given what we now know, is almost certainly alive as well, and basically all of us suspected it anyway. Same goes for Loras. Reports are he is brutally injured, but we don't know that for sure yet.
*Edited the post to add in motherfucking Barristan Selmy. Dude's probably still one of the best swordsmen in the book world.
I doubt you end up guarding the heir to the throne full time if you're not a pretty great swordsman. We've seen him beat Beric and he went pretty 1:1 with Brienne.
i mean the fact that beric cant be killed gives him a distinct advantage in some situations
