I don't plan to have my own altcoin but for those developers that are making an altcoin and hopefully it ain't like those scam altcoins. The best exchange would always be the known and popular exchanges.

They require you to pay a certain fee but that's worth it if you want to have a nice volume. Not like those exchanges that will require you no fee but their exchange doesn't have any volume at all. The best exchange would like Binance, Kucoin, Bittrex, Cex.