Is luvox (fluvoxamine) the only ssri that is a sigma agonist...and is that why it is better for anxiety..explain sigma a sigma agonist?

See Wikipedia : Drugs known to be σ–agonists include cocaine, morphine/diacetylmorphine, opipramol, pcp, fluvoxamine, methamphetamine, dextromethorphan, and the herbal antidepressant berberine. However the exact role of σ–receptors is difficult to establish as many σ–agonists also bind to other targets such as the κ-opioid receptor and the nmda receptor.Recently selective σ–receptor agonists were shown to produceantidepressant effects in mice. Best wishes.

Luvox: Fluvoxamine is FDA approved for the treatment of Obsessive Compulsive disorder. It would have to be used off-label for anxiety disorders. A sigma receptor is a non-opiod receptor that binds to different drugs. Here is an excellent abstract: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3015019/