David Boreanaz & Charisma Carpenter (Angel): Opinions vary regarding their sexual chemistry, but few can deny that they played the best of friends flawlessly.
Kristin Bell & Jason Dohring (Veronica Mars): Speaks for itself
John Krasinski & Jenna Fischer (The Office): No pair, and I mean NO PAIR of actors plays the "best friends awkwardly moving towards something more...maybe" card better than these two. The whole 30 seconds of silence scene proves that.
More to come...if i remember