I like oxygen. I'm a big fan of oxygen. But carbon's ability to do covalent bonds with a large assortment of other elements make it unsurpassed as a medium for complex, sustained chemical reactions (i.e., life) and is the medium for the only example we truly know of---earthly life.
Still, I don't want to be a carbon chauvinist, and several other non-carbon biochemistries have been proposed. I will examine what alien races would--or would not--be eliminated if the resonances didn't fall just right...next time.