30 Days of Spike Meme, Day #4: Spike's Best Friendship
Angel
Sure, Spike and Angel didn't exactly always get along, but when they start seeing eye-to-eye more, as Angel's final season goes on, it makes for a great partnership, as evidenced in certain later episodes that pair the two of them together.
Aside from being fodder for slash fan fiction, they made a great team in episodes like Episode 5x15: A Hole in the World, when they put aside their differences and tried to save Fred.
I get the impression from watching them together that David Boreanaz and James Marsters got on well too.