What is the most stable asset or currency to keep value?
Its USDT?
Lets say i have today 50$ so where i should keep it? Eur? Usd? Pounds? Gold? Usdt?
Bitcoin is not, maybe in future but right now bitcoin has too high volatility.
Well, since it is not bitcoin, then the only reliable coin that you should invest in is the gold, out of all those that you mentioned, there is still possibility that their system would eventually fade away in future, but gold is one thing that has existed for years, I mean thousands of years and it is not in this generation that it would become valueless, gold will continue to appreciate and till so many decades time, gold will continue to have its popularity, so dear, go for gold.
Tether that I would have advised you on, since it still part of cryptocurrency is a stable coin, it would never appreciate, and would remain stagnant that way for the rest of its life, while gold may still appreciate which you will get a higher value when you need it in future to do some personal stuff.