Simply put as the title says, what's been the fight of your life so far? Mine's been when I stumbled into a rather nasty sort of Draugr named Krosis. Boy was THAT difficult.

****SPOILERS**** ((Main quest))

The double dragons and the dragon priest right before entering sovngarde in the main quest

I just finished killing the dragon for one of the quests. I literally had just absorbed my first dragon "soul" and was walking towards my horse when it suddenly flees and this shadow looms over me.
Lo-and-behold, another dragon appeared and immediately tried to fry me. Keep in mind all of the other people who helped me before were already gone somehow.
To make matters worse, as I'm trying to find a good spot to shield from the dragon until my health reloaded via my healing spell a f'n troll has a random encounter at some farm and preceeds to charge after me right after. I finally manage to somehow get the dragon and the troll at a stand-off with each other and sprint away...only to watch my horse commit suicide by running into the dragon's breath.

So that was the fight(s) of my life. Epic fail