AV Slowking is so mediocre, I don't know why people use that set on a serious team if you don't need it to counter Moltres. It's weak and lacks a way to boost, has no reliable means of recovery, zero utility outside of walling shit, Knock Off practically makes it a liability at times, and half of the Pokemon it can take on all enjoy Pursuit support which AV Slowking is complete bait for. AV hardly does anything amazing to Slowking other than giving it away to switch into broken bird, but even then, they can just U-turn out and go into something like Tomb or Skuntank and proceed to trap/weaken it to a point where two Hurricanes can 2HKO it on the switch.
As Kevun and Spirit mentioned, the banning of Moltres would only help Slowking by giving it more breathing room. Sure Slowking may drop a bit in usage because you no longer need to use it on every fucking team in order to not lose to Moltres when using offense, but now you can abuse Slowking's other fantastic sets without having to worry about using it simply as a dedicated switch-in. Its offensive boosting sets are very devastating and can rip through offensive and balance teams when givin the right support, while OTR is a really fun set that makes for a great cleaner. It defensive sets are still cool, but now it can afford to run a more physically defensive spread to take on shit like Scarf Emboar, Mega Steelix, Mega Glalie, and Cobalion better, instead of worrying about being 2HKOed by LO Tres.
King is definitely not an A- Pokemon in the current meta, nor will it be any lower in a meta lacking Tres. It's still one of the best glue Pokemon in the tier, and even with Pokemon such as Tomb, Drapion, and Durant rising it viability, Slowking still remains one of the most solid Pokemon in the meta. Hell, I'd say its viability might rise a lot considering that if Moltres is banned, Cobalion easily becomes 2x more useful, and Slowking walls the living fuck out of it.
Also yeah Tyrantrum is probably A+ material at the moment. It's an amazing wallbreaker and SR setter, as well as a dangerous sweeper. What it lacks in Speed and special bulk it excels in nuking the ever living fuck out of everything with a 150 BP STAB Rock move that 2HKOs practically everything that resists it, while its decent coverage options cover everything else. It just has such an amazing matchup against so many different types of common team cores at the moment, and it's just insanely hard to deal with even with proper preperation.