For anyone who travels, or converts currencies, cryptocurrencies are the logical replacement of all other methods of funds, from gold, to various bank deposits or national cash.
A global, free to use, decentralized currency that is completely secure is a no brainer to replace all other systems. The fees to exchange cash (usually via exchange rate spread, or otherwise large fixed fees) are too great to be logical to use. The use of systems like credit cards have far too much fraud, far too high costs, and tiny worldwide acceptance (especially with small merchants).
Which Cryptocurrency?? There are thousands!
But rather than cryptocurrencies in general, we still need to be careful about the specifics and each and which ones are honest. The unfortunate situation has been that the founder and inventor, Satashi Nakamoto, is no longer guiding the development. Cryptocurrencies, including Bitcoin, are awash with non experts in this completely new area.
As a guiding rule, per the first principals of Satoshi, a cryptocurrency should:
Have a significant market cap so the price can't be easily manipulated. So we should only consider the top few by total market cap.
Be open source and free of any company or patents. This filters out coins like XRP.
Be mined by proof of work and distributed over time at a decreasing rate. This ensures honesty and fairness of issuance. E.g. The top 6 by market cap (BTC, ETH, BNB, USDT, XRP, ADA) filtered by proof of work (PoW) are BTC, ETH, DOGE, LTC, BCH, XMR.
Primarily requires a global and powerful concensus system that cannot be attacked by brute force. This is a little harder to compare between coins. Established coins are better, and BTC, ETH, LTC, BCH and XMR are considered global and powerful.
Be extremely simple to be be secure (simplicity is proportional to security), and sending payments must be completely peer to peer (on chain, no intermediatory or bank) so any two people in the world can transact between their phones with just a simple internet connection.. BTC has now too high fees and moving towards layer 2 (off-chain) payments. ETH does not follow Satoshi's guiding principal of simplicity, and goes against Satoshi's recommendation of being Turing complete (this caused the ETH bugs and fork into ETH+ETC).
So we are left with the best coins to hold and use:
BTC (Bitcoin Cash)
Hint: visit coinmarketcap.com, and filter by 'PoW' (proof of work) and 'Minable'.
Are they good currencies?
BTC/LTC/XMR beats fiat, and even gold backed fiat, in the fundamental properties of what defines a currency:
scarcity: Has far better scarcity properties than fiat or gold. Scarcity is network controlled, public, and open. Note again that a very powerful and decentralized proof of work is required for this, hence stick to BTC/LTC/XMR. By contrast, fiat currencies do not have scarcity (solely based on opaque central bank decisions). Gold has better scarcity properties, but not great, since gold rushes, new deposits, or even manufacturing gold in labs can break down this scarcity. Gold also encouraged wars and colonialism to secure deposits.
trusted: It is safe to say there is mistrust in banks and central banks. IN fact, this is the primary reason for the persuit of Bitcoin per Satoshi, even labelling so in the first block. National currencies are based on whims of secret central banks with no oversight or transparency. By contract, BTC/LTC/XMR are controller by the network, no humans are involved, no secrets are involved, it is all open source and public.
easily stored: BTC/LTC/XMR are trivial to store securely and requires no third parties. By contrast, national currencies require thirdparties and governments to store, and storage is not controlled by the owner. Keeping cash in homes is not secure due to robbery.
easily exchanged: BTC/LTC/XMR much trial to exchange completely peer to peer (no intermediatory or bank) on a phone with internet. This is easier than cash, since exact amounts can be transacted at any time, and any amounts can be help. National currencies are not compatbile between nations at all. Each country has payments systems that are incompatible, and using systems like paypal and credit cards require conversions that are too high cost.
easily divisible: BTC/LTC/XMR divisible to any size at any time, obviously far easier than cash. Online banking or credit cards can do this, however are not available to most of the world, and due to costs often are not accepted or have minimum transaction limits.
If we believe in a future of easy trade and travel for all people, not just the rich, it is a no brainer to completely switch to BTC/LTC/XMR.
Now that phones and internet are mostly universally accessible, this vision is already here.
In this vision, a nomad can buy a 50c kebab on a roadside in nepal, then that can use 15c to pickup some new chicken from an indian farm across the border. This is currently impossible, as the nomad could not change USD 50c into 60 NPR (too low, too high fees), and the merchant could not change their 60 NPR into 40 INR (too, low, too high fees). The current system has frozen out trade and travel due to costs.
The frictionless movement of money for travel and trade alone would revolutionalize world prosperity. The current systems deadlock travel and travel, undermines locals, keeps people in poverty, and undermines economic growth especially targetting the lower income. The only winners are loan sharks and money transmitters. This whole unnecessary system of sharks and middlepersons would be eliminated, returning that money to the people.