There is no "best" mixed wall in the metagame, they all serve a purpose for checking a certain metagame (in this case OU) threat. Make your question more specific as to state which Pokemon you want to check and what the rest of your team is so we can help to let your team have a good synergy. What is the rest of your team? Is it a weather team? Do you have Volcarona and a Rapid Spinner? This stuff would be useful in helping us tell which mixed wall you can use.