I like all the named vampire characters- except for seridur, who I just find plain annoying. That and he makes my game crash sometimes and always kills me a million times at the end of his quest. Janus Hassildor is fun to beat up because he doesn't die. Jakben Imbel has a very useful vampire nest in his basement, which is great for killing guards without resulting in a high bounty. He also has containers that respawn when you take everything from them, and the armor and weapon he drops usually is worth a good amount of gold when you sell it. Lord Lovidicus is fun to be chased by, but only if you're faster than him because he tends to not watch his step and plummets to his death off the side of a mountain. Vicente Valtieri is a friendly vampire assassin. It's also fun sticking garlic in his inventory because you can punch him in the face and he can't do anything.
The Gray fox - is awesome, except he always sends me places where I die about 5 times before actually Succed in completing the quest.
Lucien Lachance - cool too. The first time he appeared when I was playing I accidently killed him because he scared the crap outta me by appearing next to my sleeping character. I reloaded a previous save though to see
what he wanted.
Martin Septim - he's alright. If you rule out the fact that he sent you all over Cyrodiil to collect crappy artifacts and then just died at the end after making you go through every cave in the game, he's pretty cool.
Mahei - he's hilarious. I find it amusing how he says he's hiding from his wife and asks the player not to say anything to her about him, while she's standing right in front of him.
Mankar Cameron- I only like him because his voice sounds cool. He doesn't shut up though, and is extremely annoying.
Imperial Legion Watchman- for being extremely dumb. Idiot Guard
The Adoring Fan- its fun shooting him off dive rock over and over again.
The Gray Prince - once he finds out his father is a vampire, he just let's you kill him at the end fight