I really want to say the Mayor - I love their scenes together, The Mayor as the father figure and you could tell that Faith relished her role as assassin. Some hilarious lines also come from the Mayor in their scenes together. I also found it interesting that although a villain, and eventually a pure demon - that side of his humanity (his care for Faith) was sustained. Although Faith turned to the dark side, in her interactions with the Mayor you could also see some of her vulnerabilities, got an insight into her childhood, a sense of emptiness - you can just tell she wants somewhere to belong, as she tells Willow in "Choices" "I got someone... I got him."
But I think when it comes down to it I think Faith has the best scenes with Buffy - from their contrasting attitudes and approaches to slaying, to Faith's influence on Buffy (In "Bad Girls" which includes that dance scene, which deserves a vote all to itself) and that incredible fight scene in "Graduation Part 1". I also liked their dialogue in that confrontation scene. The almost-torture scene in "Enemies" is also very effective as you could really see for the first time the resentment that Faith feels toward Buffy,
Faith: "You know I come to Sunnydale. I'm a slayer. I do my job, kicking ass better than anyone. What do I hear everywhere I go? Buffy. So I slay. I behave. I do the good little girl routine. And who does everyone thank? Buffy."
That scene at the end when the girls both have knives at eachother's throats;
Faith: "What are you gonna do, B? Kill me? You'd become me. You're not ready for that… [kisses Buffy on the forehead] Yet!."
I think "you'd become me" is very telling - it really encapsulates the sense of doubling apparant throughout the whole series between the two characters, actually physically represented when they swap bodies in "This Year's Girl" and "Who Are You?" in Season 4.
So yes, her interactions with Buffy are my favourite, closely followed by the Mayor.
🙂