Going by what I get killed by most often, it must be general humanoids. I rarely have a problem handling dragons, giants, zombies, or anything else, with dragons way down at the bottom of the list. In fact, for the longest time, dragons were the one thing in Skyrim that hadn't managed to kill me. [small]Well, I don't remember ever being killed by wolves or skeevers, but I'd have to be AFK for those to kill me...[/small]
I used to find giants scary, since they can one-shot you if you let them get close. Solution? Don't let them. Bow sneak crits work wonders on them.
However, what I really hate is Ice Wraiths, Wisps, Magic Anomalies, and anything else using that model. I have a hard time hitting the fuckers.