Unlike the other three, Darkstone also has Mother of Darkness, a pretty good troop who is worth using. It’s just that she’s overshadowed by The Possessed King, one of the best Mythics in the game.
Ubastet and Umenath both have their charms. They both benefit immensely from Cunning, and scale well based on opponent stats. I’d go with Ubastet because he gets two guaranteed kills when used properly, has lower mana cost, and isn’t reliant on random skull creation. Ubastet also is a bit more versatile, as Umenath teams need skull generation on her teams, in addition to mana generation.
Silverglade is not a super difficult call, as Aurora is one of the best support mythics (who can fit on most teams and contribute), and Will of Nysha is a centerpiece troop that doesn’t stand out in damage or team support. I have the Kingdom here mainly because Will of Nysha isn’t as bad as most of the losing mythics in the No-Brainer tier, and Aurora isn’t quite as good as most of the selected mythics there.
As for Sword’s Edge, Champion of Anu is a quality support Mythic, whose combination of stun, mana drain, and silence can cripple a targeted troop, while adding damage that hits multiple troops without being limited by submerge. He’s definitely taken some hits since his release, as his damage has fallen behind, and stealthy and impervious troops (not to mention bless) can really limit his effectiveness. That said, I’d still use him in more situations than Champion of Gaard. CoG is a niche front slot mana generator a la Stonehammer. His armor generation and stat-boosting third trait can be taken advantage of in certain builds, but I’d say his overall utility is a bit less than CoA.
Forest of Thorns has two above-average Mythics that players would be lucky to have. Bloodwood and Yasmine’s Chosen both have the weakness of being reliant on status effects to be effective; like CoA, they are hurt by it being easier than ever before to deal with statuses. I went with Bloodwood due to his third trait being easier to trigger and better than YCs, plus my general dislike of random gem creation, but she’s really good, and has the looping potential+damage combo most people crave in Mythics.
Both Drifting Sands Mythics are a bit niche. I think the majority of people would say that Scorpius is the better of the two, since he’s the centerpiece of one of the most popular level 12 explore teams. I would counter that by saying that there are number of different teams that can do level 12 explore quickly and efficiently, and if you don’t need him for that team, Scorpius is not really a standout in any other setting. Vespera has extremely strict team composition requirements, and is a gem creator, which I tend to avoid. But since we are talking about two non-necessary Mythics anyway (if there is such a thing), why not go with the one who enables a unique team style and who’s fun to play? EDIT: Shahbanu is apparently also Explore-12 viable as well, another argument in her favor (even though Scorpius teams are probably faster).
The Orc mythics unfairly get a bad rap; while I don’t think they’re great (single targeting mythics generally aren’t), they’re far from unusable. Most places I’ve seen them discussed fail to account for how they are supposed to be used: with Gar’Nok looping and frequently triggering their third traits. With the release of Barbarian class, Gar’Nok looping should be more reliable, making these troops viable as uniquely-designed damage dealers. Gargantaur is my pick, mainly because his damage potential is significantly higher, and his Huge trait offsets any life lost from Gar’Nok (meaning he’s less vulnerable to true damage troops). That said, Shade of Zorn, when properly supported, can cast every turn and does not have to worry about being mana blocked at all, since his trait gives him mana every Gar’Nok cast.