I only play US and UK bb5, so I have limited experience in terms of comparing different nations. But I want to look at it from another perspective. Using US or UK bb5, which nation bb5 is toughest to face?
For US bb5, the toughest bb5 to face I think is KM and pre-patch RM. The biggest drawback for Montana is low range. when facing against other bb5s, if both ships have sight, you have to play defensively in Montana. in that scenario, I have to be more careful against KM because it actually have enough range advantage to hit me without me being able to hit back. RM is tough because of range advantage and low angle guns, making it harder to get out of way. But post-patch RM is not as scary, as the gun angle is upped, and range decrease. Against other bb5s, the range advantage is not so great that if they turn it, I can always hit them first by falling back, especially against 45 deg guns for USSR and IJN.
For UK bb5, I really don't feel threatened by any other bb5 in the same way that I do in Montana against H44. If I had to pick, it would be MN. Because I like to use AA/HH on my BBs, I can only go 37 knots even without armor. So MN being the fastest BB5, I worry about being rush, even though I have range advantage. I guess how to defend against rush is something I'm still trying to get better at.
Lastly, no matter what ship I'm in, when going against Montana, I tend to be more careful, as one mistake can get me kill in one shot.