I have a lot of great mods installed, but I've never installed one half as ambitious as Vilja, a mod that adds a new, hugely customisable follower (named Vilja) to the game.
A little history: Vilja was also a follower mod for Oblivion, so team that created her has a modding pedigree. (I haven't used the mod in Oblivion, mind.)
According to the mod description, she has around 4,000 lines of dialogue, though if you like, you can have her not speak at all. She levels up, has her own all-new questline, and her demeanor towards you changes depending on the conversational choices you make.
I've only played the mod a bit, and while Vilja's voice acting is definitely the work of an amateur (you can hear pops on the mic sometimes, and the mix is a little funny), she's also earnest and engaging, and certainly quite talkative. Your mileage may vary on the voice acting, but so far, I'm fairly charmed. In the readme, the modder who supplied her voice makes it clear that she's a work-in-progress, and that at some point, most things about her could be improved.
You can see a mod spotlight by Inasne0hflex on Vilja here to the side, which gives a good sense of what she's all about. I also liked how at the very start of the video, it accidentally appears as though the commentator's voice is actually Vilja's voice.
This kind of thing really gets at what makes mods so special — that a team of people put this much work into creating something so detailed and intricate says so much about the potential of mods, and why more developers should open up their games to user-created content.
Vilja in Skyrim [Nexusmods]