Personally I look at
1. Security : The most important of all since we are dealing with money. Security has to be tight and exchange has to have a swift response in case of any attacks or attempts to hack

2. Popularity: Much more popular exchanges are my preference since they have a large community around them. The larger the community, the easier it is to spot scam... Less popular exchanges can easily scam and exit with your money.

3. Team and Support: is the team transparent? Is the support swift and helpful? You don't wanna get stuck with some issue and there is no one to help you urgently

4: Trading Fees and Withdrawal Charges: I prefer exchange with less trading fees and withdraw charges plus incentives like Binance has with BNB and Kucoin has with Kucoin shares.

5: Third-party Mobile apps: These make trading easier on the go. Some exchanges have lagging websites but the mobile apps are smooth to trade with. i have traded so far with the Kucoin, Huobi, Binance and Okex apps. Very smooth and nice experience