Tether may be the most stable of the currencies, always maintaining their dollar parity value.
Is it really the most powerful? I think you have to look into Tether a bit more to understand that you may not want to put all your eggs into that basket. Also, why would you want to hold something that's only going to devalue over time? It's meant to be used as a transition currency, not really a hold. If you REALLY want to hold Tether, then just buy some USD.
People do use tether as a transition currency, but the question is for how long? If you plan to use it as a transition currency for more than a couple of months, then you are treating it as a store of value. There may be no reason to hold tether (except for anonymity, avoiding tax authorities, etc.) Tether is really Bitcoin's achille's heel.