There are some possible options as the last slide of a typical thesis presentation. I've heard of some possibilities:
A question-mark image (as the time to be slaughtered by the referees!),
A Thank You declaration (There are some negative viewpoints about these two options.),
A slide including summary of the presented ideas,
A slide reflecting the presenter's contact info (I think it does not really make sense for a thesis presenter.),
A slide including a quote (I really doubt it's the best way.),
What is the best practice to arrange the last slide, then?!