Zarok i find is the least useful of the spirits since he has 1 mediocre tactile spell and 3 offensive ones, one of which is a bit redundant (the rockfall vs underground blades). A pity the wall has so few hp...
I've always liked the cloud myself as well, a useful tactic is to fear mobs so they remain in place, then cloud them, and like was already mentioned, wait with all your units, bonus is since they are feared the enemy will also wait before defending itself which results in extra damage
the Glot armor is also vastly underestimated, it can take ANY amount of damage before fading once. ie a stack gets hit for 1 million damage and the stack has lvl 1 Glot armor it takes 0 losses, quite handy if you know what units the sadistic AI likes (poor, poor inquisitors...)
The Shoal is only useful in the early game or to tack on more damage against low lvl trash, not a great help later on.
the poison spit can be deceptively powerful seeing as you can upgrade the initial damage AND the poison damage seperatly, like was already stated the debuff from this last relatively long and coupled with some clever targetting van result in much losses on the enemy their side.
The best spirit imo is Lina, the Orb, Gizmo and the Ice Thorns are simply great. The Orb blocks ranged attackers from shooting, has no retaliation ability and can hit for ALOT of damage if positioned correctly.
Gizmo is handy but requires alot of upgrades to be really useful. Ice Thorns is fabulous against melee troops if you get alot of obstacles on the map, they draw attention, melee troops need to attack them and waste a turn and you can use it to safeguard totems if needed.
Reaper is basically a rage generator for me atm to power the others with smatterings of the soul reap skill on large stacks. Black hole is simply too expensive to be used as a mage and i've yet to select the time back ability which is a bit worthless imo...