Going by http://www.youtube.com/watch?v=VAE1e98uka4&NR=1&feature=fvwp, the arch mage is the strongest NPC easily in Skyrim. But while I haven't done the whole College questline, I've not really heard anyone say the archmage was notably hard, and I haven't seen him come up before in any (now unable to find) "most difficult enemy" threads. Just wondering what you've all been finding in regards to this.

My guess is that not alot of people attack him. Also, when you say hard npc, most people immediatly think of enemies like dragon priests, not friendly npcs

Pretty sure defeating the arch mage, who is evil, is part of the College questline.

That would be a spoiler if it was correct.
Savros Aren, the Archmage, is a levelled NPC as http://www.uesp.net/wiki/Skyrim:Savos_Aren shows. He does have an impressive spell and perk list for an NPC but just how strong he is will depend on the PCs level when they first meet him.

Strongest would most likely be either Tsun, or the three heroes.

Agreed, Tsun is one tough SOB, and he should be.

Obviously Nazeem is the strongest, I mean, have you seen how often he goes to the Cloud District?

I think the OP means the enemy NPC's titled Arch-Mage which you find at extremely high levels not the character Savos Aren.

I've never faced these NPC's so going on my experience it's Volkihar Master Vampires.

Oh, sorry, remembering what I'd heard wrong.

I am not sure if he is the strongest in a fight, but Argeir is at level 150 IIRC, making him the higest leveled thing in the game. I truly cannot tell who is the strongest NPC in terms of actual combat since I am not sure of Arngeir's Thu'um is affected by his level. If not the stronger mage enemies are likely stronger. That said the highest ranking enemy mages are ridicolously strong.

Arnegir is the strongest at Level 150 although I have no idea if he has a combat AI.

Briarhearts would be next although they can be easily killed by Pickpocketing the Heart although getting behind them will be tricky.

All of the Greybeards can supposedly kill just by speaking to you. Saying 'Dovahkiin' shakes the Throat of the World and I can imagine that they can easily Fus Roh Dah anybody off of the mountain