Why people are offering Bitfinex since it's not available for US residents? Bitfinex stopped that on August 11 - https://www.bitfinex.com/posts/216/review

The best place is Binance now. Just remember that it doesn't accept any fiat currencies, you've to make a deposit using one of their accepted cryptocurrencies.
In addition, for a trading pair which includes IOTA - BTC, ETH or BNB (Binance coin) is accepted. So it's most convenient to make a deposit using one of these cryptos, I guess. Here is a short guide on how to buy IOTA using a Binance.