Napoli. Now I say this as someone who has one, but doesn't have the Iwami. However I've never had a problem sinking an Iwami, but Napoli is far more survivable and hits like a truck.

Napoli has a relatively low skill floor and a high skill ceiling. It's a threat to everything that floats as its AP is ferocious and its HE hurts. It's got good armour, decent manoeuvrability and good speed. The secondaries are also painful when you're in range.

T10 match, Napoli and Des Moines are left. DM has circled around on flank to get to the enemy cap. I've been kiting as the remaining enemy team- two cruisers and two battleships- are chasing me to my cap. Can't outrun them. Turn into them as they reach our cap, smoke is still going as secondaries get to work. One BB died to torps, cruiser died to guns and secondaries are chewing through the other two. Last cruiser leaves cap trying to put some distance between us. I die, but we win by capping (with a very narrow margin). I don't think there's another cruiser I could have done that in.

Napoli gives the ability to impact a game much more than the Iwami, and is more forgiving of mistakes.