Bittrex is was one of the largest US-based crypto exchanges around but in 2023 they were forced to leave US and to exclude American citizens. Bittrex has USD (TrueUSD), Bitcoin (BTC), Ethereum (ETH), & Tether (USDT) and tons of other markets.
USA not allowed.
Bittrex support – https://support.bittrex.com/hc/en-us