I agree with Dagon. And also, I don't think Tenchi is all that comfortable with Washu, at least not any more so than he is with the other girls. Sure, there was that moment in Night Before the Carnival, but she still freaks him out sometimes too. And we haven't seen enough of his interactions with Tokimi to determine his comfort level with her. Sasami he's comfortable with because she's an actual child, not just in "child form." Even though she's assimilated with Tsunami, she isn't one being with Tsunami. At least not yet.
Personally though, I think he has the highest comfort level with Ryo-ohki. She's a cute innocent little girl (even more innocent than Sasami) and he seems to have genuine affection for her.
He goes out into the field for hours to collect carrots just for her and usually takes her with him. And even when he's interacting with Sasami, Ryo-ohki is usually getting in on it too because she spends a lot of time with Sasami.
