My favorite voices are Barbas, Marcurio/Faendal, Arngeir, and all the dragons' voices. I also like Mirabelle's voice.
I think the male Orc, male Dunmer, and Khajiit voices all fit their races very well. I also like the Nord voices and the Dunmer voices in Dragonborn.
My least favorite is Farengar's voice. I'm pretty sure Torygg has that same voice and I almost died of disappointment when I saw him in Sovngarde and heard him talk. I also don't like the male Breton (I think?) voice that some forsworn and bandits use, and the old lady voice that Viola/Colette have.
I like Bryn and Ulfric and Serana's voices, but I do think they're a bit overrated. They are unique and all, but not my favorite (especially Serana).
I think the most annoying voices are (in order) Tullius, Septimus Signus, Hermaeus Mora, Esbern, and Cicero. But they all have great voice actors and I like them since I think the painfully annoying (imo) sounds fit their characters. I think Teldryn and Frea have moderately annoying voices in some lines as well...
For singing voices, I really like Talsgar and Mikael/Sven, and HATE Karita. xD