[Verjigorm]

I always liked summons that get you lots of free units over time. Those are:

**scratch trolls, my bad**
Draconians
Most Elemental Royalty
Vampire Lord

Anything that makes free units usually sits in a province making free units unless I need it to do something specific.

I always look for cost effective ways to build armies. Units that make free units are high on my list.

Second highest are units or things that can be massed cheaply or units that facilitate the massing of cheap units:

Tree lords & Ivy Kings
Lamia Queens

Spells that allow me to conjure massive numbers of units with no notice allow me to store up gems and then use them as a surprise attack or defense. Any spell that conjures a large number of useful units is good for this especially if it's also cheap and I have a lot of mages that can summon them:

Seasonal Spirits
Wyverns/Drakes (given access to Dragon Master)
Shade Beasts (kinda meh, but early masses can be quite formidable)
Spirits of the Wood (I love these dryads btw. They can't heal away from home, so send a nature mage with them to heal them in the field)
Naiad Warriors (don't usually have enough Water magic for these guys)
Fire Snakes (don't usually grab these, no experience with them...)
Enliven Statues (don't usually get these either)
Reanimate Archers (these are sexy)
etc.

Again, I don't presummon large masses of these units. I wait until I know what I'm going to do with them before I whip them out because by keeping the gems instead of the units in reserve I don't give my opponent the ability to develop a counter-strategy (other than a similar summon-on-reserve action) before he has to deal with them.

Oh! I also love Great Eagles. They're made with air gems, can thug quite nicely with a luck pendant/regen ring, and great for their primary purpose... keeping spies and assassins out of your provinces. This goes for any similar patrol unit like the Simargl or Celestial Hound.

And I absolutely love summonable spy units since I don't have to spend a castle turn to make them. Black Servants and Likhos from Bogarus (Likhos are really neat spies).