If you are in the USA, it would be Coinbase. Otherwise, another exchange like Bittrex, Kraken, Bitstamp or one local to wherever you live. Another way -- if you already have Bitcoin or some other crypto -- is to use changelly or shapeshift to exchange it for ETH. The Exodus and Jaxx wallets have this function built in.
If you are in the USA, it would be Coinbase. Otherwise, another exchange like Bittrex, Kraken, Bitstamp or one local to wherever you live. Another way -- if you already have Bitcoin or some other crypto -- is to use changelly or shapeshift to exchange it for ETH. The Exodus and Jaxx wallets have this function built in.