What is the best platform to invest in crypto currencies?

Poloniex is a monster, massive liquidity & professional level trading options (margin trades, etc).

Literally never had an issue, great security practices & low fees.

I used to only use Poloniex because I got used to the UI, but I've slowly moved over to only using Bittrex. While both are seeing massive increases in usage lately, Poloniex had been giving major issues going down altogether, not executing trades, or withdrawals taking forever. Both have their flaws but I would recommend Bittrex.