Menacing, Tempest, Thunderstruck are some of my most used or favourites - great blend of resistances and supplementary stats.
Some forgotten/niche but good ones:
Overseer’s : Brilliant stats on a prefix that would see more use on characters that aren’t forced to use their headgear already (sets or +1 to mastery skills).
Chronomancer’s/Supercharged : Cooldown reduction is a powerful stat and this is one of the few ways to get on Shields.
My favs are mostly covered already, but I especially want to highlight Thunderstruck, Incorruptible, Stonehide, of Dranghoul and of Arcane Winds.
EDIT:
Also Renegade, really helps with aether res while levelling.
DISCLAIMER; I hardly spend any gametime in the endgame. The joy I get from this game comes out of building and piloting extremely bizarre and unique builds which usually aim to exploit certain mechanics and prioritize this over any endgame viability. That said;
Gildam Arcanum: for the %chance CDR- fun to build around several pieces with this suffix and hourglass devotion
Kings: for the Battle Cry. fun to build around several pieces with this suffix, especially on a support character for use in multiplayer.
Arcane Lore: bonus XP gain <3
Dancing Shadows: Increases to dodge chance!
The Grove: HP regen and Healing proc
Im sure theres more but im at work and cant really think of them right now.
As a rule of thumb, when I see an item with two or more Resistances it’s a must check for me. Because it could fit any build.
Stonehide for poison pierce and bleeding which seem to be always what I need.
Overseer’s is a good helm prefix with DA, armor absorption and stun reduction.
of Nature’s bounty for health, regen, and elemental resist.
of the Drangoul for OA/DA.
Those four things I look for and will stash. I like others depending on which character I’m playing like of fallen skies, etc. or anything that has class skills on it.
Speaking of magical vs. rare affixes, “of Alacrity” on weapons seems superior to almost every other affix, for an autoattacker
Who squealed!
The intent is actually quite the reverse.
With AoM improving affixes and a grand total of zero nerfs to MIs that I can call out off the top of my head I figured as much. I’m excited to see how many affixes end up rising to join the likes of stonehide and thunderstruck that carry an MI well over the “good enough” threshold on their own. Though I wouldn’t be surprised if most things land closer to the likes of runecarved, ancient, etc.
One thing I’ve noticed with MI armor is that instances of % damage are tragically low to the point that for practical applications you’re going to be hunting for resists as it’s easier to find the damage elsewhere (not sacrificing devotions for resistance, weapon augs/components)
Of the Arcane Blaze —> Aether Missiles (5% Chance on Attack)
Simply because I love that proc. 
It´s pretty rare though, but I still wanna build a toon around it. Wearing a lot of meh greens. Oh well …
Not as sexy/useful as “of kings” or whatever, but I don´t care.
Of Amarasta’s alacrity’s added alliterative appeal and amazing attack speed bonus.
Also any pet affix if they were to ever drop for me 
For secondary res: Thunderstruck, Kings, Flesh Hulk, Featherstep, Arcane Winds, General’s, Stonefaced, Sandstorm.
For Utility: Insight ( really good for the mana, also the chaosres is nice), Dreadlord.
Stonehide for normal res. In theory Incorruptible too. (Incorruptible is too rare imho).
I use all the magic res affixes a lot (Warding, Ordered, Impenetrable, resistant…)
I would love to use more of the affixes with a procc-skill, but usually I can’t justify those over generic stat affixes (rare or magic).
In the suffix slot i often prefer “of celerity, alacrity, attack, protection, readiness” over most rare affixes. (The ones that hit those stats, like dranghoul or fury are great, but usually don’t drop in combination with with a good prefix).
@tsuelue Like this? 
https://www.youtube.com/watch?v=r-KPjtQ_W2U (this was from a few years ago, but looks fun!)
I really like the suffixes that grant a Doom Bolt proc:
of Dark Intent
of Solael’s Malice
of the Cabal
They fall off late game but for leveling these can be really strong.
I would swear these have a different, and in my opinion much cooler, sound effect than the Doom Bolt skill too.