Notably missing from top 10 lists=
Leather Boots/Gloves- At 2 mana and Novice, these are the most cost efficient defensive Equipment in the game. They fit in almost any spellbook, they have fantastic synergy with things like Vet Belt, Voltaric Sheild or Forge Hammer. Im not sure the last time I made a book that didnt run at least 1 of these two. They are cheap, effective, and quite often an opponent isnt going to spend a valuable Dissolve to take away your 2 mana, 1 spellpoint to strip your Leathers.
Enchanters Ring- Do you have more than 2 Enchantments in your book? Do you use block, nullify, creature buffs, decoy, enchantment transfusion, healing charm, or anything along those lines? Then you benefit from Enchanters Ring. You would be hard pressed to run a book that doesnt end up saving respectable mana by running Enchanters Ring, especially in tandem with other mage's "caster" rings. As a perk, its one of the few "or" spells in the game so no matter what mage you are running its never going to cost triple spell points. (unless some weird mage pops up with triple cost Arcane AND Nature)
Demonhide Armor- Somewhat mitigated due to its Dark Mage only status, but in my opinion its the best Chestplate in the game next to Harshforge. Protection from elements is covered well enough in Elemental Cloak in tandem with other defenses/armor, rather than overstack elemental protection (which can be less useful against a rounded opponent or totally useless against the wrong opponent), I much perfer Armor+Critical Barrier. Its useful against weenie swarms, its very useful against pierce-less, but resilent zombies and helps give your mage a damage edge in a slugfest. It also adds more "bleed damage" to books based on sapping your opponent down from curses, burns and direct damage.
Personal Honorable Mentions:
Crown of Protection- Yeah..1 spellpoint, 1 mana, unlimited 2 mana castings of "diet rhino hide" on your units.
Morning Star/Vorpal Blade- Alongside Mage Staff as great cost-to-effect weapons
Supression Cloak- Gross.
Sunfire Amulet- Worth mentioning, but kinda costly for some books.
Spiked Buckler- Just a promo still, but my favorite "melee defense" equipment. Pesky combo with Dancing Scimitar.
Galvatar- Only one mage can use it, but its one of the best weapons in the game for pure aggression coupled with a crowd control option.
Sectarus- Mainly included because its one of my personal favorites, its hard to be considered a true top 10 because of how costly it is and how vulnerable it is without investing in extra protection just for it.. but its also a "must destroy" item that can become quite a game changer over just a few rounds. One of my personal favorite cards to date.
And, as mentioned, Dancing Scimitar is one of the best most rounded Equipment items in the game.. its worth it to drop in almost any spellbook.