Air tier 1:

Evade - Despite its rather large bonus to dodge, I tend to skip using this. The mana drain is actually quite high for the mediocre benefit. The only time I ALWAYS use this is if I use blindness (death) as well.

Haste - OK spell, good for champions mostly

 

Air tier 2:

Aura of grace - OP spell

Guardian wind - meh. When was the last time I faced an army of archers? Never was when.

Tutelage - Pop one on every champion. Great spell.

 

Air tier 3:

Storm - Great against monsters that rarely have lightning resistance and often come alone. Bad for sieges.

Thunderstrike - Damage too low for me to use it. Since when do you want an air mage hugging enemies?

 

Air tier 4:

Cloud walk - I don't use it because I'm mostly on the offensive.

Titan's breath - AOE prone check? NERF.

 

Air tier 5:

Celerity - same as all these spells - meh

Tornado - this is the reason you suffer through AIr3 and Air4. Tornado will win you any war against enemy factions.

 

Combo spells:

Battlecry - I love this spell, although like many combination spells its useless when I get it and great at lategame

Contagion - I love this spell, the damage potential is great.

Gentle rain - Nerf. If you get this spell you can get any conclave to level 5.

Sunder - Too situational but I guess that is ok for a combination spell. Defnitely not worth aiming for, like other combination spells are.

It should also be noted that Air is the only magic you can increase in power from technology. Ereog's Tower makes this entire element much more desirable. 

Haste: It's the best book one spell. Makes cavalry extremely effective, can be cast on most of the army by a weak mage with no other powers for a small mana cost. This spell is really good for every unit and can get you massive amounts of Init. later in the game. It just keeps getting better and better. I would like a Mass Haste for book five. I also want it to be castable on any unit so you can use it on the enemy in combination with a poison attack. 

Yep, there should be more buildings like this.

i find Thunderstrike ( + Storm) to be one of the most powerful spells that a mage sovereign can have.  If you have 'enough' mana, mage with both of those spells is untouchable and can kill any army in the game.  (well, unless opponent is doing something similar, spams archers, but I have yet to see an AI do that).  Damage part of the Thunderstrike  is meh... but, tactical teleport is amazing, ability to go from one corner of the map to the other means that no army without similar ability will ever catch up to your mage, which leaves said mage free to do whatever evil mage sovereigns do to enemy armies.

As for 'enough' mana, if you build your game around such mage sovereign, level up cities with conclave options, get Water 1 for meditation and don't waste your mana on random buffs... single mage sovereign (at least in my games) can keep multiple AIs in check (he will run out of mana eventually, but AIs either surrender or accept peace before that happens (that is part of the strategy)).

Air seems very strong to me too, air mages get a great range of spells generally

Tier 1:

Both evade and haste are cheap and have good effects although neither seems OP. Evade in combination with Kraxis' ability is a bit much though, although thats due to fortify giving a really high dodge bonus, not evade.

Tier 2:

Again, air gets a great selection. It's city enchantment provides a nice bonus to trained units and guardian wind is great at dealing with all those ranged units. Tutelage is really great for leveling up your sov too. All of these spells so far are cheap and useful.

Tier 3:

Storm is a really powerful and cheap ranged spell and is fine as is. Thunderstrike only does token damage but the real benefit is a tactical teleport and I think it's priced right as is.

Tier 4:

Cloudwalk is outstanding and it needed a price hike really - I think it's priced at the right level now. Titan's breath is also a great spell, mass proning an enemy army can really help if you have units backing your caster up, it's nice and cheap too

Tier 5:

Air only gets 2 level 5 spells, one is garbage and the other is golden. I think it's easy to figure out which is which. Air is one of the few magic schools that I'm happy to get to level 5 though, with most of them I stop at 4.

Combo spells:

I've already commented on battlecry and contagion in previous threads so I'll skip those.

Gentle rain provides a really nice bonus, air and water get a really good combo spell here. Sunder has really limited use but it is outstanding at what it does - it's basically a boss killer - find an elemental lord in the wildlands, zap him with this a few times, job done

Air tier 1:

Evade - use sometimes

Haste - a lot

 

Air tier 2:

Aura of grace - a lot

Guardian wind - sometimes

Tutelage - a lot

 

Air tier 3:

Storm - a lot

Thunderstrike - sometimes

 

Air tier 4:

Cloud walk - sometimes

Titan's breath - sometimes

 

Air tier 5:

Celerity - never

Tornado - a lot

 

Combo spells:

Battlecry - never

Contagion - haven't tried

Gentle rain - never

Sunder - a lot

Probably right, but I feel lategame it deals double the damage, or so might just be me, also less creatures are resistant/immune to lightning.
So I think its fair it got some casting time and a random targeting.
Also makes it feel way different than flame dart, and I like that.

I would love if each magic element had useful spells on each level, sadly not

Sincerely
~ Kongdej