I don't have Dawnguard, so I can't say anything about Serana, and I've never hired Mjoll. In the vanilla game, my vote goes to Erandur: He has a background (which very few followers have), good dialogue, and he has lots of comments. Ha can be a bit repetitive sometimes, though, repeating the same line about the city you're in over and over. When it comes to skills he's a bit mismatched as well - his class is priest, making restoration, speech and alchemy among his primary skills, but he mostly uses destruction spells and melée.
My favorite follower, however, is Marcurio. He has lots of really good comments, delivered in an excellent way by his VA, his skills and perks matches his spells and combat behavior, and he's awesome in general. If he only had a background, he would be perfect. Luckily for him, I made a background for him.
