Buy trading platform do you mean for spot trading or for trading derivatives and futures or just an exchange. Personally my favourite exchanges are Binance and Kucoin then Idex for decentralized exchanges. Then you have KoinPro an undiscovered giant.
If you choose the two exchange sites, I think it is a very smart choice because both of them have a very good reputation and can provide a lot of profit because the price movement at this exchange is very fast.