Useful / Meta

Cryptcanon's Vestment - Can be used in group content for other players to get their ult ready in time to burn trash or a boss. Allows supporting your permanent Magma Shell tank buddy in Infinite Archive. Might allow your (DK) PvP buddies to do some crazy stuff, such as leaping with full penetration from Corrosive Armor.

Death Dealer's Fete - A meta option for PvP sorcs and other builds. Might also work on a PvE tank.

Esoteric Environment Greaves - The single best PvP gank protection and best 1-piece defensive set, but only for (magicka) builds that can conserve or restore their stamina. Needs attention as it can be unpredictable from stamina loss and does not work against all PvE direct damage, such as PvE archers "taking aim". Can, however, be used to mitigate (Gothmau) marauder damage in early Infinite Archive arcs while sacrificing only a single armor slot.

Gaze of Sithis - It can make your life easier in solo play or PvP, especially on specs that don't want to block, such as arcanists, templars and heavy attack builds. The health regen can be stacked sky high in PvE with other sets, class passives, food, CP and Infinite Archive buffs. Nonetheless, giving up blocking leaves this item uncompetitive in the final analysis.

Markyn's Ring of Majesty - Extremely "meh" when viewed as a mythic, but good when viewed as 1-piece item. With no drawbacks it fits some PvE and PvP builds.

Oakensoul - Simplifies your build. You can only use a single bar, but the mythic compensates for the lack of your back bar. Often used with lightning staff heavy attack builds, since it permanently gives the Empower buff among other things. Not truly competitive in PvE nor PvP. You may be thrown out of the (vet) Cloudrest trial for wearing Oakensoul, due to Relequen fight mechanics. In PvP this set is limiting. Oakensoul builds can have decent pressure, but typically lack bar space for utility and burst combos.

Pearls of Ehlnofey - A meta (trial) healer mythic, typically combined with Spell Power Cure, Pillager's Profit, which are then single-barred, and a monster set.

Ring of the Pale Order - Useful for soloing content, especially the solo arenas and Infinite Archive. Some people also use it in PvP.

Ring of the Wild Hunt - Equally useful for PvP as it is for questing and resource farming. It can even make the solo arenas easier, if your build / class does not depend on Pale Order, or help your tank get from A to B, as it sometimes needs to. A quiet S-tier item.

Sea Serpent's Coil - A meta PvP item, if the playstyle suits you, i.e. you can live with the snare. Some gank protection.

Snow Treaders - Situationally strong, mostly in PvP. Used by ball groups, I believe. However the availability of root / snare removal skills, such as Race Against Time, makes Wild Hunt better for most builds that want to use a movement mythic.

Spaulder of Ruin - A meta item for (trial) tanks and healers, often worn by the RoJo healer, but might even be worn by a DD.

The Saint and Seducer - The new PvP toy. Quite meta.

Torc of Tonal Constancy - I personally like this on my DK tank. As you block and drain your stamina, you gain magicka regen. Also a good PvP option, but can feel odd. Can be your main source of sustain, but you have to build for it and learn to play around it.

Torc of the Last Ayleid King - Not bad for PvP. May, in fact, be the meta option for templars now. Suits that class and DK, due to those classes having Minor Sorcery / Brutality, buffing spell / weapon damage further.

Velothi-Ur - Meta for arcanist DDs, possibly templars, possibly in general. While you tend to break even on the target dummy, it buffs your skills over your light attacks. Some of your skills are AOE (area of effect). This mythic makes any build that isn't pure single-target stronger in trash fights, much like arcanist who wears this by default.

Niche / Nerfed / Marginally Useful

Bloodlord's Embrace - It's a tanking mythic, but is it good? Probably not.

Harpooner's Wading Kilt - Was meta for DDs when it came out, but not so much nowadays. Wear Velothi-Ur or something else.

Lefthander's Aegis Belt - Another item that trades away a core combat mechanic. You might try this in PvP, because it mitigates AOE damage that a dodge roll will not, but in the final analysis I do not find the tradeoff better than a standard dodge roll and, therefore, a waste of an armor slot.

Malacath's Band - A meta PvP item on release, but nerfed. Does anyone still use this? I doubt it.

Mora's Whispers - Not quite good enough for anything. There isn't really a build pattern where this would justify taking a head / shoulder slot as far as I can see.

Stormweaver's Cavort - The problem with this mythic in a hybrid and spell-crafting meta is that there is hardly any use for it. If you're really into buildcrafting, you might like to tinker with this, but I haven't made anything outstanding with it yet. The only genuine use has been for my vAS+2 Z'en / MK front kite, allowing me to keep stamina permanently low while using magicka for the frequent dodge rolls necessary in that trial.

Thrassian Stranglers - Pretty niche and nerfed, but still a lot of damage. Some people make this work in solo PvE and even PvP.

Why do they even exist?

The others