Excluding Serana, who's your favorite Dawnguard follower? My money's on Celann - he's a glorious tank. Deck him out in full heavy Dawnguard armor, hand him a shield and an axe and let him go. I've hardly ever seen him stagger and have witnessed him taking down many different vampires with ease. He's not a bad shot either - giving him a crossbow and a supply of bolts will definitely yield results. There's also something implicitly trustworthy and genuine about him. It's very nice knowing that you have a vampire-slaying veteran on your side. Durak is a fairly good sniper too and I've found that he doesn't get in the way terribly much or get hurt often. Having an old Orc Oblivion-bent on avenging the deaths of his wives with you is a great confidence booster as well. Ingjard is an all right tank in the right type of armor with a pretty good two-handed skill. It's also amusing when she gets bored and hums a song. Beleval is awful. Her best skills are two-handed and light armor, which do not mix well at all. She gets staggered all the time and hardly does anything useful. It baffles me immensely that she's a Bosmer with virtually no skill in marksmanship. Agmaer has just about the same skill set as Beleval and the same problems. If he was good with a crossbow, he'd be useful. But, it appears that all that practice with Isran was for naught.