It's pretty much fact that Logen is among the baddest dudes ever to set foot in the First Law world. His most impressive victories are in LaoK (singlehandedly kills The Feared and an Eater on separate occasions) and Red Country (lets Glama Golden beat on him before smashing him to a pulp, then later escapes from a completely impossible situation when he's cornered with Savian).

So who could actually beat Logen, in his prime, in a 1 on 1 fight, assuming the Bloody Nine comes out (which almost always happens in desperate situations). Gorst seems like a contender, but remember that he seems pretty evenly matched with Whirrun during their fight, and Whirrun has nowhere near the reputation in the North as Logen does.

In my mind there's only one real contender, and possibly a second. The first, and most obvious, is Shenkt. He hits so hard and so fast that he rips people apart like his hands are high-powered modern weapon projectiles. It's true that Logen managed to take down an Eater, but Shenkt is exceptionally dangerous even for one of his brethren.

The second contender is Stranger-Come-Knocking. He seems like he's an even larger dude than The Feared, but is smarter and level-headed. Unfortunately we don't really get to see SCK fight much in The Heroes, but the way he's depicted seems like it'd be a fair fight.

I'm curious about what the rest of you guys think.

Also, Logen would smash Gregor Clegane to a pulp, no question.