I think there are probably two types of builds that would make the most sense for the highest difficulty solo raid content, and that's what I am focusing my theory crafting on at the moment.
First and foremost, I understand this is a HIGHLY custom version of the game where class balance/power is significantly different, but since I am operating off pure hypothetical and the basic info I've seen in videos, I will first audit out classes that I feel will not fit into any "highest difficulty build".
1.Druid. This is the easiest class cut for me, as anything they can do, other classes perform better. I understand there are going to be some changes to make them more viable, but I would rate them as "unessential" at this time. Even their general utility is limited since there are now 4 SoW classes and additional methods of world travel to diminish teleports value, especially since they can be swapped to as a secondary teleport loadout.
2. Ranger. I know bows are getting significant beneficial changes including melee range. I don't know enough about their changes to say, I am definitely using this class in classic for the hardest content. Exception might be the t-staff bow in plane of sky, if its truly unresistable by any npc target, this may move them into usable tier for hardest content.
This reduces the 560 possible class combinations down to 364!
This is still a pretty large list, the next criteria for hardest content in my theory crafting is the ability to SLOW the mob. This means one of the classes in each class combination Must include: Shaman, Beastlord, Enchanter, Rogue, Bard.
Running a quick table array IF statement in excel resolves this problem for us.
This reduces the combinations down to 280.
We run the same type of formula to verify that there is a tank class of Paladin/Sk/War.
This reduces the combinations down to 135.
We want to ensure there is a healer and a tank class in every possibility.
A SUM formula with COUNTIF resolves this to ensure SK/PAL/War = exactly 1 for each three class combination.
This reduces the combinations down to 120.
We use a similar formula to ensure there is a cleric or shaman included for healing.
This brings our possible combinations down to 42!
We are finally down to the nitty gritty, we have met the criteria of having exactly one tank class, one form of slow, and one form of significant healing.
Some final audits are to eliminate redundancy in classes where the overlap may be less desirable. So I will be eliminating: Shadow knight + Necromancer / Paladin + Cleric / Shaman + Beastlord.
This brings us down to 33 viable "Max Raid Difficulty Solo" Specs down from the original 560 combinations.
Of these 33 Specs, I listed 22 as "Top Tier" For Solo max difficulty raid content, but included a tier list for all:
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All that being said, I'll probably do something like Beast Lord/Berserker/Bard just to play some new classes I've rarely explored, and level up a raid combo later.
I imagine many combinations I eliminated will potentially be better than any remaining class combo I listed, I imagine the class changes/world balancing will wildly change results.
Also just because I got bored of the exercise, I did not explore the pet stacking dynamic as I do not know how pet aggro and chain summoning will work yet.
Edit: I was referring to mage pet as being tied for strongest, but not with enchanter pet which charmed pets would be the strongest, but with summoned pets I imagine DPS optimized is closely matched with beastlord pets.