I think the highest number of damage achieved was actually by a priest that mind controlled a boss/mob in Naxx.
It hit for over 100,000 damage. the quote was "If only it had crit"
EDIT: Correction! The largest one-shot was by a PALADIN from release, that 1-shotted Lord Kazzak with a reckbomb collected from over 3 hours of duelling and storing crits. Reckoning was hotfixed within 24 hours of the event.
The ONLY reason he hit for so much was because it was set up.
Raid of enhancement shammies + alliance = 4872934 crit
Shamans don't crit for that much at all in BGs because they usually will only have 1 stormstrike (10% base mana, 10 sec CD, 12 sec duration) on their target. And even if they time it right and stack it up to 2, the casting time of Chain Lightning is well over 2 seconds.
"The moment instructor razuvious dies in naxx, his understudies become bugged for a few seconds, and take aorund 900% more damage per hit. thats all it was."
But the pally in the video I posted earlier with 1800 reckoning charges stored...assuming his 2hander is pathetic and half of them miss, the minimum damage there would be at least 200,000. The game can only register so many hits at once, and since there's actually another bomb from seal of righteousness it seems like there was more than 1 hit on Lord Kazzak. Additionally, that's not from a single number, but rather from over 2000 numbers combined in one swing. If each hits for a 100....yeah, it's the highest amount of burst damage from a single attack to date. Pity it got nerfed.
The priest's 142k hit is the highest single number recorded for damage, although it's been done several times with probably more damage.
If there's a raid of shamans you get a raid to counter it.
What about that Nurfed Warrior who went through that area in the Burning Steppes solo ... that was overpowered, warriors unfortunatly were nurfed to kittens after that.
Ah, back in the day before super gear was everywhere. I miss those good ol' days. OLD SCHOOL FTW.
But that was quite some time ago (august 28 2005), back when Valor was considered the best gear there was, and warriors that could do that kind of damage were feared above all to be sure! I remember being so proud to be able to knock one down to 50% health when I was level 45.
But just for entertainment, here's a paladin that went to stratholme with a bunch of healers and buffs and managed to pump out a consistent 1300 DAMAGE PER SECOND!
There's no need to fear paladins for the reckbomb anymore...24 hours after the Kazzak kill the charges were capped to 4, and at 2.0 the entire talent was changed so reckbombing is completely a thing of the past. What you see in the above link is probably the best a paladin can do in terms of damage now.
Here's an old school classic warrior video by Pat...it's pretty old too, and was during the week where I think warriors were bugged and got 1 extra talent point. But if you wanna see insane pvp numbers, not much will match this one. I remember during a few AV's I was in a while ago Hellhammer used to do this kind of thing all the time. http://video.google.com/videoplay?docid=184118310320652017&q=pat+high+warlord
My conclusion about "overpowered", then, is simply that ANY class, given great gear, buffs, and a little bit of luck, can be utterly insane. People just don't seem to believe me.
On a more practical scale, the only thing I'd consider overpowered without buffs is the mage, but will be quickly evened out with the coming of Burning Crusade. Stand at a distance or unflagged, ZHC, fireball, POM fireblast. 3k, 3k. 6k damage in 3 seconds on any target. Essentially, being able to 1-shot any player under 6k hp. Happened to me in my last video.
Just a little while ago Melko posted a video he made that features the trinket mage.
But pvp balance is within and between groups. There's more to pvp than just damage. What use is having insane damage if a pally/rogue combo stunlocks you to death?