This depends on how the market is. The only fiat that is profitable to buy is dollar because you can convert cryptocurrencies to dollar at a very low charges on exchanges. So, it is like converting to cryptocurrency.

During bull run, people buy altcoins, that is why altcoin price increase could be double of bitcoin, so it is good to convert from bitcoin to altcoin at the time.

If bearish market begin, people convert back to bitcoin, this is why the price of bitcoin do not decrease like that of altcoin in bearish market because people demand for it.

During bearish market, some people just convert their money to dollar as I said in the beginning and buy again when the price is low.

Aside dollars, I can not advise someone to convert to any other currency except exchanges support the currency and if the currency value is not much decrease like some countries money.