Liquid currencies.

This concept is especially important when trading on Forex, since the success of trading largely depends on the currency's liquidity.

Liquid currencies are those that are in constant demand and can be easily exchanged for other currencies at any time.

Demand is the primary indicator of liquidity; the faster you can sell your currency, the higher its liquidity.

The most liquid currencies on Forex at the moment are:

The US dollar is extremely popular and therefore in high demand. Its liquidity is confirmed by the lowest forex spreads on currency pairs in which it is included.

The euro is the second-largest currency in the foreign exchange market by turnover. While it was previously used primarily for settlements for European goods and services, its applicability has now expanded significantly.

The Japanese yen is used in virtually the entire eastern region and for settlements for goods produced in Japan.

The British pound is a widely used currency in financial transactions, gaining popularity due to its stable exchange rate.

The Swiss franc is arguably the most reliable global currency, used by individuals to store savings and banks to hold foreign exchange reserves.

The Australian dollar is primarily used for settlements for gold and agricultural products produced in Australia.
 
The Canadian dollar is sometimes called the oil currency, as Canada has been a major exporter of black gold for many years.

Less popular, but still offering sufficient liquidity, are currencies such as the New Zealand dollar, Hong Kong dollar, Chinese yuan, and Russian ruble.

However, for Forex trading, it's best to choose the most popular and liquid currencies, as they allow for the use of any Forex strategy due to their low spreads. Another advantage is that when choosing, for example, EURUSD, your order will always find a counter-order, and the broker won't have to act as the other party to the transaction, significantly reducing trading risk.