Here's my thoughts on ES/FB armor. Keep in mind that's for 95 ES specifically, things can change with lower ES like 75-80%. IMO there are really only 3 choices for her.
Enigma:
Currently I use this because it allows me to have a bit more life/mana/damage than other setups, and I don't have to worry about FRW. Overall just a solid standard choice with lots of str (i.e. life/mana) and a little bit of %PDR, which is not that important for ES Sorc but still something. But the main reason for me to use this is because personally I have a good inventory for Fire Sorc (5x high life skillers, 1x FHR skiller, high life/mana SCs etc). Enigma allows me to just use my best charms and be done with it rather than worrying about FRW/life/mane/damage too much.
Arkaine's:
1.07 Ark has massive life and an open socket (Sol/Mal whatever) so that's certainly good.
But I wanna repeat my argument for 1.10+ Ark on ES Sorcs as well, in particular against phyiscal damage dealers. It can get up to 22 iPDR with Sol socketing. That's a big deal with 95% ES because so little physical damage actually reaches your life pool.
For example say a Bowazon deals 600 PvP damage per arrow. With 95% ES, only 30 damage will hit the life pool. Now 1.10+ Arkaine's with Sol reduces that by
73%.
In comparison, 1.07 Ark with Sol has only
23% damage reduction against that particular arrow.
Obviously it's just one example (albeit a realistic one) and other armors are more benefitial in other situations, but just wanted to illustrate the power of iDR on ES Sorc.
Viper:
Apart from the FCR, an idea with Viper is to stack MDR. That's Mal in helm & armor and using String of Ears... Arach is not needed for FCR.
That's because of an MDR bug which to my knowledge is still in the game and works like this: Say you have 50 MDR, and some physical damage dealer uses an item that includes just a little bit of elemental damage. This is the case in many cases (Highlord's, Faith, Ravenfrost...). Now let's say your MDR negates 20 elemental damage, then the "leftover" 30 MDR will actually be applied as physical iDR. So basically MDR doubles as PDR in such situations. And Viper is generally a great armor.
(Btw IIRC the same applies in reverse with iPDR with Arkaine's and socketing Sols, but does nothing vs casters with pure elemental attacks.)
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As for CoH or Ormus: Nope. MDR > resists for 95 ES Sorc and otherwise CoH is worse in every respect than the above.
I see no place for Ormus either because I want + all skills (BO, ES, Warmth, Meteor all benefit).