I'm using poloniex and it seems to work for me right now. I haven't tried all of those out there, but I have used a few of them and poloniex seems to be the most user friendly so far. No guarantees because we know what happened with mtgox and cryptsy.
And cryptsy had problems long before anyone knew about them. For all we know, Poloniex could have problems no one knows about. There's a long history of btc businesses failing and losing depositor money, and this is a new industry. You're really dealing in faith when dealing with any online bitcoin service.
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