It depends on what coin you are going to trade, one coin is not available in all exchanges. But if is available on many exchange better choose which exchange it has the most volume and liquidity by that your trades will be executed faster. I prefer to trade on binance It is top 1 on volume and liquidity and they have an option to use BNB as fees and you can have 25% discount on fees, while 5% discount on Margin trading.
I agreed, even the exchange is one of the top and yet your getting any profit in it, that's remain useless at all.
Now even if the exchange is not on top but it gives you profit at most of the time I could say that platform is one of the best
anyhow, in my opinion only.