For now the better exchanger in my opinon and had less problem is Bittrex.
but no matter what,they're good as exchanger no more.even though the're an exchanger regulated under law,
they still can not guarantee your money and your assets.
well,
that is the better exchanger for now,and if you're a newbie and want to know how to do the trade.
i recommend you to start a demo on Hitbtc.com,
you can try how to do the trade in there without deposit anything.
never trust an exchanger,and always find an opportunity from different exchanger.
because sometimes you will see a difference in price between one exchanger and another,take it as an opportunity,
and do the arbitrage.
Bittrex,Kraken,Hitbtc,Bitstamp,Bitfinex,Coinexchange,Novaexchange,Cryptopia
Most of them offer you Bitcoin and Altcoin pair,
and also Bitcoin and Fiat/USDT(1 to 5 offer this).
The most common bittrex and Poloniex, and reviews of good are many and pleasant for users. Yes, and different coins can be exchanged.