No matter what kind of search we have to do, whether its an invalidaiton search, patentability assessment, freedom to operate or any other type of search, most of the people I have seen are always looking for some new high-end database for patent searching, such as Delphion, Micropat, Patbase, Patent Cafe etc.
No doubt these databases are very good, but a good database not just determined by the no. of jurisdictions it covers but by the algorithm it uses.
As a search strategy always use Google Patents after you have performed a search on these big shots databases. Since the algorithm used by Google patents is completely different it will provide you with different results as compared to what you have already analzyed in Delphion and the like.
So as a rule never stick to just one database.