I've been using Liquid.com.

On Liquid you can enjoy trading with freedom and get the support you need with up to 25x leverage. Take full advantage of the tightest spreads around.

Fund your account and power your trades with your choice of crypto and fiat offerings, including BTC, ETH, USD, SGD, JPY and EUR.

Amazing user interface too!

Check them out here: https://www.liquid.com/margin-trading/