My thoughts on this
Baseline Humans - Physically speaking I always thought of the Scylvendi or Galeoth as the biggest and toughest of these. We all know Cnaiur is the Most Violent of All Men.
Dunyain - Essentually Uber humans. I mean we know Cnaiur was a beast to all Men except Kellhus who is at a significant advantage.
Nonmen - We know Nonmen are stronger and bigger than humans, the ones left alive are also hardened from age and experience. Mekeritrig loses to Kellhus and resorts to Sorcery so I rank Dunyain tougher than the Nonmen. There are tales that Ciogli the mountain broke a Wracu's neck, although if this is a hero's tale you have to assume that it must have been an extraordinary feat and so rank Wracu's higher.
Sranc - Basically in a 1 vs 1 Sranc are probably the bottom of the list, smaller and weaker than humans.
Bashrag - These could be the underdog here and take it, I mean they are designed as tanks.
Skinspies - We've seen a lot of their martial expertise, higher than humans.
Wracu - We've not seen much Wracu but from what we have seen I think theyr'e probably top of the list.
Inchoroi - We've not seen much of their fighting but I think they might be fairly tough.
So with that in mind:
1. Wracu*
1. Dunyain*
3. Bashrag**
3. Nonman**
4. Inchoroi
5. Skin Spy
6. Human
7. Sranc
*I'm just not sure if the Wracu could ever hit the Dunyain.
**Again I just can't call it. I expect the top Ishroi would smash an average Bashrag but maybe a big Bashrag could kill a low tier Nonman.