If you are denoting market for exchange then exchanges like Binance,Bittrex and even cryptophia are safe because I had exprience in trading those sites but the trading fee is also a part to consider about because some hae high fee structure so you need huge capital to cover those expenses,in my opinion Binance is better place for trading because it has less fee.
It is always important to choose exchanges that are widely used and of course, I will always vouch most especially for bittrex and binance, with some soft spot for Kucoin exchange as well as I believe it also has some huge potential to be a solid exchange in the long run, and the fact that volume is increasing by the day on it.
There are so many things to always consider when it comes to trading, and for high volume trader, fee is one important factor after security, so in that case, I guess Binance wins the day.