I love my MN BB's. They are fast, have high damage and awesome AA. The downside though, is that they are low ranged and have low submergence (ability to take damage) and health.
They are an aquired playstyle, however I would honestly recommend going one of the original 4 nations first.
The KM BB's are fantastic in tier based rooms, and they have the best BB3. The BB4 is more than competitive, and the BB5 is awesome (but has a few bugs). The BB6 is one of the two most popular in HA's.
SN aren't junk, but the BB5 and 6 need a bit of adjusting. Again, they are an aquired playstyle.
I would honestly recommend going US; They are a good nation to start with.