Obviously, it is entirely up to you. But I will go for MN first. 

MN: Easier to Grind, good gun damage, good gun accuracy, very fast BB(42 knot), can play Armored or 12 shell for EBB4 at Level 90. BB6, highest damage in the game. Gun range is the lowest.

RM: Best Reload, quite Fun at low levels, level 90 EBB is very very good but BB5 is rubbish. Worst range at BB5 and second worst range for BB6, and damage is quite low. Bad accuracy if your gunner is not Level 120. Always need carry 2 pack AP Shell due to low angle. 

Obviously it is cool to be boring these days and to farm GB with Moskva and Charlemagne. So, the answer is MN or SN.

I'd suggest KM or IJN.

MN or UK

but if u want Challenge, grind CV ^_^

KM and SN BB6/BB7s are the best in the game for competitive play.

It's up to you.

RM has some really nice ships at most tiers up to BB4 level, including the excellent Capitani Romani, the insane Caio Duilio / Conte di Cavour, and one of the best EBB4 in the Project 41-B. However, once you reach BB5+, you have arguably the worst ship at every tier from then on.

MN has much less impressive early ships. Lower BBs especially tend to be either slow, lacking firepower, have low range, or have terrible spread even with high level gunners. Sometimes all of the above. A few are fun, but only with higher level crews. Even the PBB is a pain at level, though it does have arguably the best PCA. Once you hit BB4 though, you start getting fast ships with good speed and firepower (or at least tight spread to make up for lower gun count), including the Richelieu 1951, which is up there with its RM counterpart as being one of the most OP ships at BB4 level. And from BB5 on, you have the best aggressive ships in the game, though the apex is at the 6.5 (Bourgogne) level rather than the rather lackluster BB7.

If you're going for HA ships, MN is a no-brainer. Otherwise, it's the question of the better later ships vs the better earlier ones.