The Zero-Crossing rate threshold for a voiced/unvoiced decision
Plain ZCR criterion is not enough for robust and accurate voiced/unvoiced separation. That being said, your threshold should be adaptive, there is no fixed threshold that works well for all speech waveforms. It depends on the approach you follow but a statistical threshold should do the work most of the time. You can google your question to find relative papers and see if their approach suits your purpose.

Just a note, ZCR is not the best choice for pitch estimation - it just gives a rough estimation.