Axii. All day every day. buff the conversation perk ASAP, as you not only have the prompt to do so all the friggin time, but it's a nice little batch of xp each time, and can get you out of some annoying fights later in the game. Also, on higher difficulties turning an enemy on his friend he will often one shot them (or be one shot'd), so yeah.
I'm still playing skyrim. Well, technically I am. But with all the mods it's an entirely new game. I would rather there be a donate option for mods that clearly took hundreds of hours of work to complete, but I don't use Workshop for Skyrim anyway; Nexus Mods is the way to go.
Micolash by far. I ended up beating him on the first try, but it was the most tense, exciting and unique boss fight in the game, IMO. The fact that it mixes in PvE while tracking him down was a nice change from 'enter arena, mash boss till dead' like all the other fights I've seen.