|"The Last Supper"|
In "Home, Part 2," Baltar is deeply questioning the existence of his Head-Six. He's fed up with her cryptic messages and offers of virtual sex: "You're not my fantasy, anymore." This sort of thing, of course, has always gone well for him (does he not remember what happened in "Six Degrees of Separation??"), but, instead of disappearing for the entire episode and then showing up again when Baltar regains his "faith," she morphs from her nakedness into a T-shirt and sweat pants. She completely drops her persona, even to where her voice is different - not the sultry cooing that I'm used to - and displays emotions that Head-Six never does. She laughs with and at Baltar, and she outright mocks him with a sarcasm that really does get you wondering if she is indeed simply a manifestation of Gaius' subconscious, "frakking with his mind." She encourages him to "wake up and smell the psychosis already," a clarity and directness that Baltar both appreciates and fears.
At the encouragement of this hostile version of Head-Six, Baltar meets with Dr. Cottle**, requesting a brain scan, hoping to find a chip that would explain where she came from. He desperately wants her to be Cylon technology, and therefore easily explained, just so she won't be the "latest version of [the] little voice" inside his head. Head-Six messes with him while getting the MRI, touching him in his special place and causing him to instinctively jump, prompting Cottle to yell, "Can you stop going crazy in there?" Of course, nothing turns up, although Cottle says that he can stare at the scans all day if it makes him feel better.
What makes this my favorite is that it's so different from the other sequences of their astral interactions. It really makes you start wondering if she's a figment of his tortured imagination or if she really is a messenger from God. Perhaps it is his strident attitudes toward religion that made him a perfect candidate to be a tool by which the One True God operated, who knows? But it seems like this, especially after Baltar discovers that Helo and Caprica-Eight (she's not yet Athena in the episode), is the place where the scientist finally begins to truly believe.
* For some reason, I simply cannot find an image of Six in her non-overly-sexual outfit, and I'm way too lazy to do a screenshot of my own.
** By the way, Doc Cottle is probably one of my favorite secondary characters. His little sarcastic quips and complete refusal to quit smoking while caring for his patients just makes me laugh.