Last time I checked, destruction-mages need near -100% casting cost to be viable.
Thus conjuration + alteration is more likely to encounter.
Either way, you can drop cost from spells down to 0% for any school... so cost is not an issue, really.
Back on topic:
Conjuration, Alteration and Shouts are the most powerful "schools" of magic.
Even with 0 cost(which some consider an exploit) illusion's pacify is just as powerful as alteration's mass paralysis while illusion is also a more versatile school. And pacify is targeted. Illusion gives quiet casting as well as muffle and invisibility, so you can set up most encounters completely in your favor before they even begin. With illusion + conjuration you can kill many groups of enemies entirely without revealing yourself to them.
I agree with Odd Hermit's ratings, other to say that the one Conjuration summon he thinks is too powerful is to me the only adequate one. The others die too easily. Fire Atronachs and Thralls die like wimps, and in fact even the Storm Thrall does, and I am only on Adept st level 50+.
Yeah the atronachs are definitely too weak. It'd be fine if the dremora lord was supposed to be the "ultimate" summon, but seems like they wanted it to focus on atronachs or undead but the dremora lord is hands down the best. Part of that is due to twin souls and other atronach perks working for dremora lords, though.