Currently nobz themselves are decent, so units with multiple big choppas are great. In addition you will want some cheap nobz to take the first casualties, best option right now would be nobz with two choppas, you can just switch some arms around on the
AOBR nobz for that. If you are afraid of
GW removing that option, just don't glue them yet, the arms on
AOBR nobz will hold without glue for a few years or so
In boyz mobs, you probably want your nobz to wield
PKs, so they can deal some damage to vehicles and kill characters. A big choppa isn't far behind though.
In general, I'd suggest getting a box of nobz to add to your collection. It probably has enough bits in it to provide for 10 nobz and then some. You'll also get a killsaw and a powa-stabba out of it.