well the easiest of them all i would say its uk. Very good damage output from the guns a lot of vessels to choose from plu the extra support slot which makes it extremely easy to have your support at bb6 when ur bo hits 120.

Now from MN and RM i cant really say as i dont have a MN bb yet (still at dd).

The RM 0-60 lvl is prety fun. A bit frustrating to start with but once u get to cl lvl with a good pair of gunners u will own in blitz. Just got to AD lvl and it was really fun upto now.

Yes, UK is probably the easiest line to start with for the reasons stated above. 

MN is a different style, but still fun to play. Great AA and mostly front guns from BB3 onwards, so it will give you a good rushing ability while staying hidden from the enemy. Although it has the lowest range, it has the strongest power.

RM is a lot like UK, but if you want to start easy, go UK. 

so rm is tougher than uk?