Everything depends on what user wants to buy. Best way to buy altcoins are popular exchanges like Binance, Kucoin, Bittrex. But you should not forget about small exchanges like crex24 or hoo.com for example. Sometimes projects does have enough funds to pay for listing on a huge exchange, so they are forced to choose less popular one. That is why, dont underestimate small exchanges also. Most important - before depositing funds, google a bit and try to find if the exchange has any issues with users previously.