I'd go with oldpop here, 16" guns penetrate pretty much anything the game throws your way and more barrels means faster ladder aiming, leading to earlier and more hits. I'd love to build an 18" H-class or Yamato rip off, but couldn't find a justification considering the price. As was said before, carriers and planes are the major threat past the 40s, and carriers sink to 12" guns just as much as 18".
The 'Montana' layout of 12x16" is ideal, for me. But then I like a 12-gun battery better than 8 or 9 and am willing to drop caliber an inch or two to get it... My past games show you can win battles with massed 14" batteries (if you have DDs for the coup de grace) and a good 15" gun will work perfectly well also.
One 'house rule' I play by is to limit caliber increases to 2" or a +1... so I won't go from 12" to 13" but will move up to 13"+1 or 14".