kraken always seemed ok never a problem there  but leverage offered is quite low.

okcoin is the worst, regularly took positions and came back to find them closed out and money gone (due to margin)  even though the price hardly moved.  this exchange is definitely 100% working against traders to take positions against them in order to force margin closes against them.  avoid it.

bitfinex is second worst, some real dodgy stuff going on there as well  and the fees are quite high. better than okcoin but only just.

best solution is dont trade leverage with bitcoin guys, you cant trust any of the exchanges there are scams on all of them.  only use leverage with reputable & regulated providers.

if you trade leverage with bitfinex or okcoin you are playing in a rigged game

best option - deposit to bitstamp and buy & hold.