Two global enchantments that I try to cast in (nearly) every game: Gift of Health (GoH) and Forge of the Ancients (FotA).
GoH completely eliminates your main worries about aging. It also allows you to keep those super-elite units rolling without them being crippled by afflictions. Finally, the extra HP in your dominion is very useful for protecting your researchers and support mages from the AI's beloved Seeking Arrows....
FotA is a delightful way to bulk up on magic items in a hurry. The gem savings are astronomical! And the easier forging is nice. (Even if they did nerf it a bit from the Dom1 glory days.) D1 Revenants can spam Skull Mentors with FotA active. 'Nuff said!
Other nifty spells:
Aim / Eagle Eye - Don't overlook these. The coolest spells in the world won't do much if they never hit anyone.
Summon Earthpower - Added Earth magic
and more reinvigoration? Yes, please!
Nether Darts - Best when combined with a Spell Focus, a Ring of Sorcery, or some other item to smash through MR.
Shark Attack - The last word in underwater combat.
Astral Window - Cheap information gathering. What's not to like? Cast this spell at every opponent's capital ASAP and re-cast as needed.
Winged Monkeys / Manifestation / Earth Attack / Infernal Disease - Ranked roughly according to their reliability (with Infernal Disease at the top of the list), these spells are great for whacking nasty enemy commanders and/or stranding their armies.
Gateway -
The solution to how to reinforce your conquering armies.
Faery Trod - Cheaper than Gateway, but often just as useful. Don't ignore this spell!