IDEX and Kyber network are one of the largest DEX. Though Changelly advertises itself as a DEX, it's not. The closest DEX alternative to Changelly is airswap.
Binance DEX is a cool new option and with much documentation than any others. Decentralized exchange gives you the feeling that you are always in control of your money.
Oops, I didn't know that Changelly wasn't a decentralized exchange, I guess they still deal with your funds as you send it to them and they hold it for a bit just to make sure it confirms.
I've seen IDEX in action, and isn't it more of a hybrid between the two? I don't think it's fully decentralized, but it isn't centralized either, and I've heard of Kyber previously.
Actually, this isn't a platform, but localeth is a DEX. You use your own private keys for it.