Hate to say this now...
Myoko to Mogami - Hands down... Once 5.3 comes out.
Unless they see the massive impact the AFT/BFT changes are going to have on this ship, and its longer shell travel time than a Premium (Atago) - This ship will go from among the Top of the pile in 8v8 competitive play to the bottom with the New Orleans.
Now take this with a grain of salt; Though currently our clan and a few others are taking part in a Clan Wars-ish event with a tournament format. Ive yet to see a single one run an NO; Almost every one runs a Mogami/Atago with a few using MKs. With these proposed changes shown in 5.3 this will without a doubt remove the Mogami and promote the Admiral Hipper with its exceptional Anti-CA/CL and the AA it will pick up. This leaves the 'best' or 'most viable' ships being almost borderline pay-2-win, or pay-2-advantage even. (Atago/MK)
This doesnt even strictly apply to the CAs; Only one match so far has another clan used an Amagi and it was the first dead BB every game last weekend. Two weeks ago we tried to make an Amagi work, and it loses too much fire power at the risk of exposing its frontal citadel area. The Tirpitz on the other hand is going to pick up AA, has torps, and a non-existent citadel; Which propels it into a position to be taken over Amagis even further. Which has been the case in every single game so far; Everyone runs at least one, or two of them with an North Carolina.
Heres another interesting tid-bit as well; Every clan so far runs double Benson with either Fubuki or the Blys... With the Lo Wang coming out, it will shift the way current 8v8 play is based purely because of it having Sonar to deal with other DDs that rush the caps to mini-brawl it out... It has 68kts torps and aforementioned sonar to deal with the opposition sitting in their smoke to deny capping as the team moves to support them.
Mogami will bring NOTHING to an 8v8 match that a Hipper will do better in every scenario other than attempting to chase down DDs with 155s;
Atago; More HP, Faster, Faster Rate of Fire, Faster Turret Traverse, REPAIR PARTY, Lower concealment to Stealth Torp (900m window);
Mikail Kutuzov; Further Range, Higher Rate of Fire, More Health, Higher Chance of Fire, SMOKE that allows stealth fire for a brief window.
New Orleans; Ugh... Nothing to really say, its the absolutely worst CA in the game and brings virtually NOTHING to a match other than AA; Our currently 8v8 Tournament requires 1 CV per team, though even then a North Carolina will easily be able to defend itself in 5.3 more-so than it already does now.
Admiral Hipper; Higher AP Damage, Faster Rate of Fire, Faster Shell Travel, Higher Firing Range, Faster Rudder Shift, More Health, Best Sonar available, Better AA, Faster Self-Defense Torpedoes.
Now as for the Mogami, what does it have going for it... Honestly very, very little. Its got the tightest turning circle+Rudder Shift time. Rather bouncy armor even vs 406mm shells which is honestly one of the larger advantages it has other other T8 CAs. Though that is literally all its got going for it. Once 5.3 rolls out it will be the shortest ranged CA by over 2km (15.1km) which is really looking bleak for the ship as a whole.
TL;DR - Last Paragraph sums up Mogami as a whole. It will now be fairly on par with the New Orleans.