What makes a cryptocurrency a legitimate currency?
To answer that question you have to consider the characteristics of money, which are: durability, portability, divisibility, uniformity, limited supply, and acceptability.
Cryptocurrency is infinitely durable. It's "portable", in that you could hold it in a wallet like Mycelium on your phone and you can transport it anywhere in the world without an intermediary. It's infinitely divisible and the smallest unit of a Bitcoin is typically regarded as one satoshi, or a hundred millionth of a Bitcoin (that much is impractical to spend now, but it's still relevant).
It's uniform because counterfeit Bitcoin is impossible - the transfers of Bitcoin are verified in the blockchain to solve the double spend problem, which is an important function of the blockchain.
The supply is limited - the amount of Bitcoin distributed is the reward which is given to miners for mining a block, which was originally 50 Bitcoin and halves approximately every four years (currently at 12.5 Bitcoin for a block), and a block will always be mined approximately every ten minutes. Fiat money is not limited as it is no longer based on anything with a limited supply and can be created at will by governments and banks, which is a key thing that sets Bitcoin apart and is the reason why some people regard it to be "digital gold".
As for acceptability, it's not accepted everywhere but as it matches all the other characteristics of money, this can change.
what makes a coin such as bitcoin, xrp, ethereum, etc. really worth it?
Bitcoin is worth it because of what I just described. Ethereum is a platform for other people to create blockchain tokens, so it basically exists to spur other people to create original things. Research smart contracts to find out more about how this works.