I think the 25,000+ year old Draenei mages disagree that corruption is a set deal when it comes to magic. If you approach it with the absolute mindset of power, chances are you will become a necromancer or warlock, etc.
However, that isn't the only route to power, just the easiest one. There are mages that can cast several incredibly powerful spells in seconds. Spells that can wipe out tens, hundreds or maybe thousands of people in seconds. Archimonde destroyed Dalaran, a city with incredible magic shielding, in seconds with one spell.
Magic is, essentially, everything in WoW. It is your ability to change the surroundings. However you do it depends on your style. Shaman techniquely use magic in their spells and totems. Druids cast spells. Priest prayers are essentially spells (note that this only refers to their active spells, not things their respective deities do for them without asking, such as the magical shield that protected Tyrande from the BL for a month or more that even Archimonde couldn't break.)
I'm sorry, but melee classes have no reasonable reference point for claiming to be the most powerful. That isn't to say they aren't necessary, without them the casters could be swarmed by angry masses of demons in an instant. But, aside from Grom defeating Manoroth (also a melee unit, not a mage), they really haven't done much.
For the most part, your average mage is going to be more powerful than the average Warrior. And the Absolute mage is going to be more powerful than the absolute Warrior. Note that I am using the term "mage" for all magic users. Their specialties kinda form a rock-paper-scissors effect.
For example, the Eredar Warlocks don't really have any defensive spell techniques. So, a mage that uses mostly defensive counter-spells will be superior where a 50/50 defense-offense mage may fall.
A Druid can adapt to a huge variety of situations and exert great power, but if a Warlock manages to literally kill the very elements around them, his tools will be severely limited.
A Mage who did all their studies learning how to channel magic from Ley energies may not cast the strongest spells, but they may be able to maintain a solid shield until their opponents are exhausted or their comrades break through the enemy lines.
Azeroth's saving grace against the BL is its variety. They have a very active true diety (Elune) many active Demi-Gods, a rich supply of elements, a well balanced ley-system, guardians that prevent chaos from erupting (dragons) and a huge variety of races, which was rarely seen in any documented account of other worlds the BL has conquered. This diversity is the only way Azeroth can fight back.
The BL works by sending thousands of demons to their deaths just so they can inflict some damage. They have what may literally be an infinate supply of warriors, so the only way to stop them is to cut off the portal supplying their troops and destroy their leaders.
Think about it in terms of the RTS games. If you only send Orcish Raiders and Bathandlers to attack the enemy base, you probably won't win. However, if you send an endless, nonstop flow of the units, they may make slow headway.
But, if you send a good amount of units that complement eachother (healers, ranged, melee, magic, flying, siege, etc.) then victory is much more likely.
The four favorite units the BL use for invasion are:
-Fel/Doom Guards, melee and melee/flying units, respectively.
-Infernals, mindless melee units with an initial ranged strike.
-Eredar Warlocks, the only real ranged fighters.
-Felhounds, to take care of mages (much more useful when stealthed than on a battlefield where the mages aren't near.)
They have no healers. The only ranged attackers they have are terrible at countering spells. They just use quantity.
And well coordinated magic DESTROYS them.
Okay, sorry for the semi-off topic ramble.
If the topic is, "Which Class is Most Powerful Lore-Wise?" It HAS to be a Mage or Warlock, depending on your point of view.
The most talented mage in the Universe would defeat the most talented Warlock, because of utility.
The average Warlock will defeat the average mage because of the copious amounts of damage they can inflict in a short time; if the mage isn't up to countering every spell they are done.
In a real-world fantasy scenario, a spell doesn't take away half your life. Either you counter it before it takes full affect (hopefully before it even hits you) or you die.
If I just make everyone on the opposing lines explode spontaneously, I think I am more powerful.
Also, there aren't "Resists" in the real-world setting. A resist is when you screwed up your spell. A good caster would know how to work their magic so that is never an issue.