Binance is most popular exchange, but it is crypto-to-crypto. If you need to deal with both fiat and crypto, go to CEX. It is also quite popular and easy to use. You can trade either crypto for crypto there, or fiat for crypto (https://cex.io/eth-btc). Also, Coinbase is a large cryptoto-fiat exchange. You should also take into account your location because some exchanges have geographical restrictions.