Choosing a coin
A short decision for a question people overthink.
The desk article for a decision people overthink. In most cases the answer is Monero, and the exceptions are specific.
Default to Monero
If privacy is any part of why you are here, use Monero. It keeps the payment off a readable public ledger, and most entries default to it anyway. This is the answer unless a specific reason overrides it.
Use Bitcoin when forced
Some vendors take only Bitcoin. When that is the case, use Bitcoin, but do not send straight from an identity-checked exchange to the market. Route through a fresh wallet you control first, or swap Bitcoin to Monero inside a wallet like Cake before depositing. That breaks the trail between your name and the market address.
Use Litecoin for small orders
When the purchase is small and Bitcoin's network fee would be out of proportion, Litecoin settles faster and cheaper. Two entries accept it.
Use Ethereum only when you hold it
One entry takes Ethereum. It buys convenience when that is the coin in your wallet, not privacy, because its chain is public. If privacy is the goal, the decision circles back to Monero. Cross-reference the payments topic.