Monero is perhaps the best option imo. Here’s the official page about it, but basically:

• not profitable to mine, so most miners are enthusiasts who want the coin to succeed
• privacy-focused - basically creates a ton of fake transactions to mislead snoopers
• relatively popular - seems to be the most popular coin recommended by privacy enthusiasts (e.g. Mental Outlaw, he even gives a discount on his store for Monero)
• not popular among speculators - they mostly stick to the big ones (BTC and ETH), as well as new startup coins
• low cost transactions, fairly short transaction window

It’s far from ubiquitous, but it’s popular enough that if a place accepts any crypto, there’s a good chance they accept Monero as well.