http://uesp.net/wiki/Skyrim:Lord_Harkon
Easily the strongest we have encountered in any TES game. There's also http://uesp.net/wiki/Lore:Opusculus_Lamae_Bal_ta_Mezzamortie whom was the very first Vampire. Her story explains what vampirism is, and it's purpose.
This is also worth a read, I think: http://uesp.net/wiki/Lore:Immortal_Blood
Well, the Count of Skingrad is the only example of a vampire I can think of who gained power and prestige in spite of his condition.