This is gotta be a common topic... Even a poll... My vote goes to Mjoll the Lioness and Teldryn Sero. The feature you'll notice I'm aiming for with these two is dialogue. Nothing is more game ruining than a robot of a follower who says one or two lines over and over again and simply does whatever you say. I want a character to follow me, not an algorithm.
I don't use followers. But in the rare times that I take somebody with me, I like to take a follower of an opposite playstyle to my own - so if playing a warrior, I'd take a mage or an archer. Melee combat gets as crowded as a doorway full of Lydia sometimes.
Mjoll's attitude was refreshingly upbeat, so she stayed with me longer than most. I had Serana with me for ages because I didn't ever finish the quest, so I'm incredibly sick of hearing "What do you need?" over and over again.