Brynjolf - I could listen to him say the phrase "larceny's in yer blood" over and over again.
Esbern - Love those wise old man voices. It helps that he speaks a lot of truth, too.
Ulfric - Very deep and kingly. Makes me want to side with the Stormcloaks just to hear him speechify.
Gormlaith - It's not so much just her voice as the hilariously overwrought and poetic way she says everything. Picture her ordering lunch - it's hilarious!
I like Serana's voice a lot (because, c'mon, it's Laura Bailey), but I'm not sure how much she fits into Skyrim's milieu. She's too...normal.
In terms of "good" voices, there are loads of voices who fit the characters very well, chiefly is Ulfric (musclebound idiot indeed), Cicero (absolutely fucking mental screech), Paarthanax (wise world-worn sage), but I don't like any of these characters.
Neloth is probably my favourite, only because of his weird pitch changes. "hmMMMM?" "I could dig up a stAAAF or TWOOO for YOOOU." Love it.
My all time favorite is Hakon One-Eye. It's just...so manly. I could just stay in Sovngarde and listen to him for dayyys. A few honorable mentions are Karliah, Isran ("SLEEP IS FOR THE WEAK"), the entire male condescending voice (AKA Belethor and Lucan Valerius) and Galmar Stone-Fist.