Truthfully, there is not one school of magic better than the rest, each has it's own abilites and weaknessess. Conjuration is good for mages, as it allows you to summon a monster for your enemies to beat on, and for you to escape. It's also good for spellswords, as you can conjure your own weapons. Restoration is excellent all around, as it restores health, Fatigue, and all attributes. Illusion is good for weaker charactors, with it's ability to hide the caster, and improve how people feel about them. But also it can confuse enemies, and light your path. Destruction is a must for any magic user, with it's devestating elemental spells, and ability to damage health and attributes. Alteration is one of my favorites, as it can open locked doors and chests, as well as shield you from damage of all types. Mysticism was taken away with Skyrim, and it was a good choice. With only the usefull powers of seeing living creatures through walls, and soul trap, the rest were just parlor tricks. Although, Telekenisis was usefull with the Welkynd Stones. This is just a brief run-down of the schools, and it's up to you to decide what you want, depending on the situation and your personal preference.
In game? Destruction then Conjuration than alteration than Restoration (Only for healing though)
As Destruction is useful as Skyrim is a combat based Rpg
Conjuration for making dead followers or spirit followers, magelight. You get the idea
Alteration as i can oakflesh, alter my silver to gold. Etc
HOWEVER IN LORE. (Like if it was real life in elder scrolls)
Alteration hand down, it would be the hardest school to learn due to the powerful effects. But i can become rich, change the law of physics and make myself invisible, stronger or anything. Be really risky so learn fast mate. Don't try to upgrade your skill like in game lol.
Conjuration. Especially on high difficulty levels, with the same amount of magicka to cast three or so fireballs you can conjure a dremora lord that does an incredible amount of damage on most enemies... And draws the enemies attacks away from you.
But of course, having enchanted gear that negates the magicka cost of destruction spells and dual casting with impact perk is easy-mode all way through the game.
I think it would he lazy to not master all schools. Destruction has damage, conjuration has summoning. Restoration has healing. In otherwords, doing a damage mage would be very resteictive because the whole point of mages is that they can do lots more stuff then warriors with swords.