A few options...
* You could edit the Site - Search widget to put the Wiki category of results at the top of the list
* You could pass in a parameter to search results each time to filter by the Wiki facet (but the person searching would have to click to remove this filter if they wanted other results)
* You can tweak the decay within the Score Configuration for content quality (with Telligent Community 7.x) to make the decay more aggressive for content types other than wiki and make the wiki decay very conservative. You can do this at the group level if you have wikis in certain groups that you want more prominent than wikis in other groups.