MN it's UK with more speed and better reload but without UK thoughness and reprate. Also small ships = harder to hit. At the end = Charlemagne which is great ship for GBs.
US is great dmg and reload but not that much range and speed. Bigger ships = easier to hit. At the end you will get Nebraska the best all around BB6.
If you liked UK than KM will be very hard for you (I went KM after getting UK BB6 and it hurt). At the end you ve got Kaiser with the best range in game but not that great all around (especially for GBs).
I would pick MN.
Or you can go with Italian line which is still a bit OP = quite fun and easy.