I've yet to find anything better than the Sogou one. It easily passes the "nhm" = "你好吗？" test, and has a lot of slang and shortcuts built into its dictionary. It learns commonly used phrases/names and will prioritise them in the list of conversion options after you've used it for a while. The Sogou dictionary is so good that Google was caught copying it a few years back.

Just be careful with the installer; much like every other piece of software put out by a Chinese company, it'll do its best to fill your PC with a crapload of useless social gaming apps, antivirus tools, and other assorted spyware.