Bittrex is one of the leading cryptocurrency exchanges by 24-hour trade volume. While the platform's liquidity and variety of trading pairs are best-in-class, the user experience leaves a lot to be desired. If you're looking for a specific altcoin, Bittrex might be a good match, otherwise, I'd keep looking.
Kraken offers great volume, responsive customer support and many advanced trading features. My review left me with some technical concerns and it sounds like I'm not the only one. If you're just starting out with trading cryptocurrencies, maybe start with a more beginner-friendly platform.