Krosis.
I was only maybe level 14 at the time, I swear that bastard killed me at least 10 times. That fight was tough considering all he had to do was get three consecutive hits and I was down. I ended up having to spend more than half the fight cowering behind pillars and healing myself.
The guy was annoying as hell too, anytime I actually did get into range to hit him with my weapons, he just kept kiting and killing me with fireballs. Only way I could even hit him was to charge into range, use my slowing shout, hit him with a double-power attack and then run for cover before he could turn me into a red smear adorning the rocks.
I got so desperate I just started trying to hit him with the Wabbajack, turns out that's what won me the fight. I hit him with it, and he turns into a rabbit. So then I just bum rushed him, slowed time, and hit him with everything I had. Turns out it wasn't enough because once I hit him he turned back, and I couldn't kill him in one rush so I had to polymorph him again to get close enough and finish it.
Though the first time I fought a Blood Dragon was tough, but I wouldn't really consider that a fight, since after killing me 3 times I realized I had no chance and just ran halfway across Skyrim, Benny Hill theme music playing in my head, until it got distracted by some bandits and I managed to lose it.