What do you want to do with your mage?

For powerful tactical damage mages, you want Water IV for Blizzard, or Fire II for Flamedart, or Fire III for Fireball, or Greater Necromancy for Horrific Wail (Empires only), and then you want to climb the Evoker line of the Mage tree. Blizzard and Horrific Wail in particular can be army-killers if you have a mid- or high-level mage casting them. Death can also be good if you have Death Shards to power up Drain Life or if you go for Death V to get Kill - if your damage mage is going for Kill, focus on getting the Prodigy traits rather than the Evoker traits, because if Kill works, the target dies, and Evoker doesn't do anything for that.

For powerful strategic damage, you want whatever gives Starfall, ?Vetrar's Howl (the Water spell that does AOE damage to armies), or Pillar of Fire (there could be a couple others that I'm forgetting, as well). Then you want to have plenty of the appropriate shards (Fire for Pillar and Starfall, Water for Vetrar's Howl; Starfall might also benefit from another shard but I don't remember right now).

For powerful tactical support casters, Water I, Air I, or both is practically all you need, especially if you have a decent number of Water or Air shards, because Haste and Slow are two of the best support spells you can have. Life III is good for Growth and Shrink, and Wellspring's a good army-healing spell, while Death can be decent even without lots of Death shards, as taking even just three attack off of a late-game unit (~20 physical attack) with Wither is effectively a 10% physical damage reduction, and Curse or Mass Curse can greatly benefit an army that doesn't have a lot of magic damage on its troops. Graveseal and Infection are also great spells, if you have them.

For powerful strategic support casters, Death I for Wither is good, particularly when backed up by lots of Death Shards. Whichever levels of Earth and Water give Tremor and Freeze can also be good, since those stop armies in their tracks, but there may still be a bug with them where they get divided across stacked armies, and due to the way turns work they don't really last quite as long as you'd expect them to (without the bug I mentioned, you can expect them to last 1 turn less than the listed amount). If Grip of Winter works, that's another good one. Unit enchantments can also be useful, though I don't usually make use of them at the strategic level (Tutelage, though, is good for champions as long as you keep them fighting something). If you want to play with unit enchantments, I'd suggest turning them off in periods when you aren't fighting, because they are a significant drain on your mana economy, particularly in the early game.

Summoners basically have to go up the Summon line, since there isn't any other way I know of to get summons, and past the early-mid game I'd rather be using the tactical summon spells than the strategic summon spells in most cases (there are exceptions - for example, Air Elementals are great scouts, and there are circumstances where I might want to cut costs or don't have the mana income for continually using tactical summons). I personally prefer my summoners to combine into a summoner/support caster, because generally speaking it takes fewer levels to get that combination to work well than to get summoner/damage caster to work well, but with Empires (Horrific Wail comes from a trait in the summoning line, and that's one of the best damage spells for killing armies you can get) or sufficient numbers of monsters or sufficient amounts of time invested (Quest Map loops, endless wars farming AI armies, farming lair spawns, etc), there are exceptions.

Other things that can help make your mages feel more powerful:

1. Set the magic density to high at game start - more shards means more powerful elemental spells and more mana for casting them
2. Set the monster density to high - mages tend to become more powerful as they gain levels, while warriors/defenders/assassins tend more towards a strong start and an average or weak finish; this is especially true for Fire Mages or Damage Necromancers - Horrific Wail and Flame Dart deal damage based on the caster's level, and can get up to an 80% (100% for Flame Dart) damage bonus if you get all the damage traits.
3. Put Meditation on your cities and lean more heavily towards Conclaves - high level Conclaves can provide an extra 2 Fire Mana or 1 Water Mana, and have the best mana production of any of the city types, as well as the potential to have the most essence
4. Create a custom faction with some combination of the following traits: Decalon for spellbooks, Enchanters for scrying pools, Adepts for early access to shard shrines and extra starting mana, Flesh-Bound Tome for some useful spells (Cull the Weak and Death Lash) and a debatably useful spell (Consume - gives 200 mana but permanently destroys a shard) and a not-so-useful spell (Candlecloak), and Death Worship for several very useful spells (Dirge of Ceresa, which is only really useful for Empires; Corrupt, which I think can be used to produce Life Shards for Kingdoms, though Life Shards are only really good for healing; Graveseal, which is good for anyone trying to produce a support mage who can greatly boost the damage output of an army; and Infection, which is the curse spellcaster's dream come true if you can get it to stick)
5. Binding and Cult of the Hundred Eyes can be used to give a faction a magical flavor but don't really do anything for spellcasters specifically (as a warning, Binding doesn't do anything with Life Shards, so if you play a Binding Kingdom, one of your shard types won't give you free units, while the most powerful spider - the Bound Hoarder - from Cult of the Hundred Eyes requires an Empire-only technology - the Black Quire)
6. Give your mages initiative bonuses (daggers, trinkets, and a few General-category traits; also can be acquired from one of the spells you get out of the Magic research tree, or from one of the spells an Air mage can pick up), mana cost reductions, spell mastery bonuses (curse and damage casters mainly, as blessing casters and summoners don't need to overcome spell resistance unless they're casting outside of their specialty), spell damage bonuses (damage casters, primarily), and critical chance/damage bonuses (damage casters again - funnily enough, critical hits work for magic, too)
7. If you're playing with Henchmen or Sions and have access to the Decalon for spellbooks, these guys can become great mages because of all the initiative and the free levels you can provide them on training, but they have to go in for one or two of the elemental spellbooks, for summoning, or for one or two of the mancy traits, as the elemental adept abilities which Henchmen have are not generally as good as the real spellbook, and Sions don't have access to any of the elemental adept traits; additionally, Henchmen can provide an Empire faction with limited access to Life Magic if you want to play a custom Race of Men Empire
8. If you want your sovereign to become a powerful wizard-king, consider building up other champions, Henchmen, or Sions as masters of each of the elemental magics you can get, and then casting the spell that lets you take one level of a magic school from that unit at the cost of killing them (for efficiency, I'd suggest building your sovereign with Life/Death magic, taking Decalon as a faction trait, and picking Men if Kingdom or whatever if Empire, then head towards the spellbooks/sion/henchmen techs, give your sovereign Air/Earth/Fire/Water I using spellbooks and building fast-leveling Sions/Henchmen to pick up each successive level - remember that if you want your sovereign to get Water Archmage, he needs to get Water I either through a spellbook or by sacrificing a caster with any level of Water Magic, and each successive rank in that magic school requires the sacrifice of a spellcaster who already had the rank you're trying to give your sovereign - no sacrificing mages with Water I to give your sovereign Water III - and the same applies for other schools of magic)
9. There are a couple of sacrifice spells that convert city population into mana or sovereign health - the one for sovereign health can be used to fuel lots of Paragon spells to boost your other champions or to make your sovereign into a really tough unit, while the sacrifice for mana can be used to help fuel major spells like Curgen's Volcano
10. Turn up the quest density at game start to increase the likelihood of finding the quests for Mana Blast (along with Kill, potentially the most powerful single-target spell in the game) and Mana Shield

Everything Joeball said

Also, just buffing your units and putting enchantments on your cities already provide huge bonuses. Your mages fully earned their sabbaticals behind the frontlines!