Not the toughest ever, but one of the more surprising and unexpected battles happened to me just now.
I've just started a new character, he's currently Level 6, a Redguard with skills in One Handed, Block, Destruction, Heavy Armour and Restoration. I'd just cleared Bleak Falls Barrow and was making my way to Whiterun when I came across the random encounter spot in the turn in the road (just after the wolf).
Knowing it was a random zone, I slowed down a little to make sure I couldn't see anything, before putting my steel sword (superior) away and carrying on. Until, I got blasted with two lightning bolts out of nowhere as a Khajiit in steel armour flashed across the screen. I had lost more than half of my health already, and managed to dodge another three long-range lightning bolts before getting myself into cover behind the fallen tree on the corner. He was darting around, trying to get a clear look at me again, while I healed and equipped my lesser ward.
Like a total bad***, I jumped over the log, dodged a bolt, cast my ward and closed the gap. I evaded
another bolt and unequipped my ward, leaving my off-hand empty to block. After all of his bolts, he was down to just using sparks, which still seemed to hurt more than normal, but I managed to catch him with a bash before laying into him with my sword with a flurry of bashes and power attacks.
Not very often I get surprised on Skyrim these days, but it can be pretty fun when it happens