Sorcerers, Mages, Warlocks, they are all essentially the same thing with minute differences. And, of them all, I would say Warlocks are the strongest.
Why?
The very nature of the Warlock is power. They rarely bother with defensive spells and focus all of their energy into doing nothing more than burning you alive. In the War of the Ancients books, one of the favorite spells used by the Warlocks literally dehydrated you to the point that your body turned to dust in seconds.
Only high ranking Warlock would have true control over demons, so I won't mention that. Although, it is worth mentioning their ability to summon an Infernal in the form of a meteor from the sky, which causes massive damage and then gets up to cause MASSIVE damage.
It is also important to note that Warlocks draw their power directly from the Shadow and Sargeras, so it never really runs dry or is even strained. Mages rely on the magical forces around them and, while some become true experts at molding the energies, many more powerful mages existed at the time of the Well of Eternity than in modern Azeroth--there was just more magic to go around.
However, to the true magical genious, it is a tough call. If we are talking the AVERAGE mage vs. AVERAGE Warlock, I vote for the Warlock. But, if it was Malygos vs. Archimonde/Kiljaedan/Any Warlock other than Sargeras himself, then I would vote for Malygos. The sheer utility of his spells would be breathtaking. He could counter everything the Warlock threw while doing damage to boot. Hell, he can ERASE stuff. I mean, literally. The very molecules are just GONE.
But, he is kinda insane now... And kinda hates all of the mortal races... And... yeah...
So Warlocks > Mages until you approach the most phenomenal.
Also worth mentioning, in areas where nature is alive enough, Druids are crazy powerful, if prepared. They can do just about anything, and they do it in ways traditional magic cannot counter, which is invaluable in war.
[EDIT]
Rustle, this is a discussion of WoW Lore, not Mythology. I mean, if info from any source is available, the discussion kinda wouldn't work in context.
Oh yeah, and your comment about mages being killed by guys with swords? That is actually incredibly rare in WC Lore. Mages/Warlocks/<insert magic user> kill other Magic users. RARELY will a footman or knight get close enough to them. Think about it, Mages can fight far out of range of arrows and such, sometimes so far away that they have to use magic to amplify their vision. I mean, Illidan wasn't anywhere near the Frozen Throne when he nearly destroyed it via a spell.
Mages worry about other Mages. Some train in Melee combat, just in case (IE Rhonin), but that isn't an issue on the battlefield. They would only need that if they were escorting someone and got jumped, etc.
Why wouldn't they need it? Mages can teleport in the lore much easier than in WoW. They can go farther distances (sometimes easily from continent to continent) and do so without using much magic, without a reagant and very quickly.
If a swordsman kills a Mage in the lore, the mage was either incapacitated or stupid.
And, lore-wise, Priests were never really Warriors until WoW. You had Voodoo Shadow Priests, but they were more the "headhunters-bring-me-sacrifice" type, and not suited to actual war. Shadow and Light ARE the two magical chaos forces. One is pure destruction, the other creation. And, guess what, the Burning Legion is winning. Maybe they haven't taken Azeroth. But, they have destroyed what may literally be a countless number of worlds. The Naaru are trying to organize an army of the Light, but that doesn't mean all their Warriors USE the Light. They want mages, shaman, hunters, rogues, and everything else.
Every warrior of the Burning Legion uses the shadow. Their bodies literally burn with (shadow) flames fed to them by Sargeras.
No melee warrior stands ANY chance against them, unfortunatly. Just being near them burns them alive. And I would say Rogues are the weakest class lore-wise. At least hunters have range, but a Rogue is practically disabled in every way on a battlefield.