There are many things I like about Poloniex, for security requires authentication of 2 factors for withdrawals and to use the API, the vast majority of customer deposits are hermetically stored cold, Poloniex has audit programs every day of the year and block all activities it considers suspicious, allows trading by margin and leverage and is regulated by FinCEN.
Poloniex is a reliable exchange, is a U.S. registered and regulated company, offers a clean and sensitive design, allows trading with more than 100 altcoins with commissions of the lowest in the market, deposits and withdrawals without charge.
Poloniex has 4 base cryptocurrecies are used to exchange which makes the exchange experience more comfortable and inexpensive while other platforms only offer 2.
I recommend this platform for the amount of cryptocurrencies offered to its users, have greatly improved security after the attack of 2014.