You need to assign a per-hop behavior marking to give your traffic some priority level. There isn't a suggested value, because this codepoint really varies, based on your individual network needs. You might want to select a codepoint, monitor the results, and adjust as necessary. For more information, see the Use codepoints to assign per-hop behaviors page of the QoS information center articles.
What per-hop behavior marking do want to give to this differentiated service policy?