I was away from crypto for years and now I'm trying to get back into the game. I was really STUNNED that there is no exchange, even after many years since bitcoin started, that we can just deposit USD and start exchanging altcoins. I hope I'm wrong because this would be the single biggest hurdle for altcoins to be widely adopted by general public. There are just too many hurdles to jump over just to START trading.
Binance is the most popular one nowadays. The volume is enough high. You can try bittrex as well. I like it also. If you need a decentralized one check dex integrated to waves platform, it is not yet useful so much but safer for sure.