Hands down, I think the best writing and acting goes to animals. The Spiders in the Gobbo base, the various Cats and Dogs across Baldur's Gate, Scratch and Owlbear Cub - it all fits *SO* well. Even the disguised Redcap Sheep in the Swamp and the happy frog are hilarious. I found it especially cool to find out from a *cat* that there was a devil in the Brothel, or learn some of my own background from Steelclaw.
Perhaps they are too 'main character' but, Raphael and Auntie Ethel are personal favorites.