For me I love Tara singing to Willow. Those moments will live for eternity in my mind. When she talks about how it is Willow that has happened to her that has made her this wonderful thing. It is a moment I will never forget.
That was so beautiful.
My favorites have got to be these:
In "Who Are You?", when Willow and Tara do the spell.
In "Who Are You?", when Tara says "I am, you know. Yours."
In "Hush", when they move the refrigerator with their hands. That was so good.
In (as you said) "Once More, With Feeling" when Tara sings to Willow.
In "Family," the scenes with the two of them, especially the beginning. This was a pretty emotional episode for me because I could really relate to Tara and her family background. The end scene with Tara and Willow dancing to Melanie Doane's "I can't take my eyes off you" was very beautiful.
In "Seeing Red", their conversation before the credits. It was so funny to see them laughing and being able to enjoy themselves.
In "The Gift", when Willow and Tara hold hands so that they can part the people so that Spike can save Dawn.
And there's so many more that I could say.