I would probably say backbone. It just seems less figurative to me.

Both are clear metaphors but I personally wouldn't use either one. I would search for a more descriptive word that specifies why the person is valuable: engine, brains, genius, magician, creative force.

I wouldn't be so flattering, but they both sound OK. Backbone sounds better than Rock, in this sentence. You could also say things like: You're the champion of our company. You're the star of the company. You're an inspiration to all of us. I have actually been to a show with 6 entertainers, and told one of them "You're the star of the show!". Although I didn't word it like that. While shaking his hand, I said "Very good. Star of the show." This may sound like broken English; but when you're really comfortable with the language, you're not shy to speak more sparingly, in the interest of clarity and humility.