Depends on what you want to do with it? Your question would be like me saying . I need a truck at should I get. it depends on what I need to do with it and where I live
In our church: Pastors use an iPad to remotely control keynote and powerpoint presentations and displaying sermon notes. AV folks use iPads to remotely control pro-presentor presentations. Adminstrators use iPad apps for music team scheduling under planning center. Musicians use the iPad to display music and words.
I am sure there are more. Regarding what is "best"? It is the app that does what you want.
Will the iPad work with the overhead projector connected to a Dell computer running Windows XP ?