I need only one thing-my private keys.
All the other stuff,like liquidity,low transaction fees,good customer support,etc. are important as well,but you know the saying."If you don't own the private keys,you don't own the coins."

High level of security is the most important thing because many centralized has been hack, and second one is they should be have insurance to avoid member lossing money incase the exchange get hack or others bad issue. Crypto exchange also should be have fully regulation to avoid ban from the goverment. Alot of centralized exchange been operated but less decentralized exchange, even though alot of trader want to use decentralized exchange because we have the private key so that mean we have full control with our funds.