I have used Bittrex and Polo with no problems, though Polo got really slow recently because of heavy trading. You can also try out Bitshares DEX (decentralized exchange), but not as many people use it, so trading is not as efficient.
I'd recommend holding BTC, LTC and ETH, and you can also look into Factoids FCT, XEM (NEM), and Dash.
RE: How Steem has helped me advance in the Cryptocurrency world.