The BSV Exchange primer
This is a question that comes down to where you are located and what type of funds you plan to use to purchase BSV.
Assuming US, bittrex.com is the going to be the best direct USD to BSV option as they are licensed in most states. Circle may do something after the sale/release of poloniex.
Again assuming US, until November 1st, 2019 poloniex is a great indirect option via coinbase. You can purchase BSV like this: purchase USDC or BTC on coinbase, after those funds are available, transfer to poloniex. After those funds are received and credited on poloniex, purchase BCHSV on either the BCHSV/USDC market or the BCHSV/BTC market. Then transfer your BSV coins to the wallet of your choice. (I highly recommend centbee, exodus or simply.cash for experienced crypto users and handcash for beginners. Moneybutton is excellent for smaller amounts).
This same indirect method can be used with kucoin or coinex (instead of poloniex) though various daily withdraw limits may apply.
If using coinex, you could also purchase BCH on coinbase (instead of USDC or BTC), transfer those to coinex and purchase BSV in their BSV/BCH market.
Outside the US, flotsv is the most bsv friendly and highly trusted exchange, though liquidity may be smaller as it a newer exchange compared to the others listed.
Also outside the US, hitbtc and bitfinex also have BSV markets but while those exchanges have been around awhile, there have been more reports of problematic experiences. Your mileage may vary.
Not financial advice.