The best leverage exchange is definitely Bitfinex, very liquid and among the most used .... it has a fixed lever at 3.3 that you can not modify, but it is much better so
Then there are Bittrex and Polinex exchanges that you can leverage up to x5 and you can choose.
BitMex can operate with lever from x5 to x100
My advice is to start with very small amounts, only with more liquid coins, to understand the operation and never use lever higher than x10; you risk getting hurt and burning all your tradign account!
May I ask if you have noticed or experienced sudden "spikes" when higher leveraged, which easily could take out your position? It seemed a commen practice in Forex when being a bit over leveraged that you could experience a margin call with a sudden spike against your position out of nowhere.