What is the best "last slide" in a thesis presentation?

The last slide will typically be seen for some minutes after you finished talking – until you jump to some other slide for addressing a question. This is something that you should use. If you ended your talk with a summary (which is a good thing in most cases), leaving that slide gives the audience opportunity to reflect on your talk, remember what they wanted to ask a question about, or just let your central messages sink in. If they do not want to do this, but focus on the questions, they are not distracted by anything new that you didn’t talk about.

The main exception is if you find it difficult to orally convey that the talk has finished – in that case a thank you slide or an any questions? slide may be the lesser evil and save you from a few seconds of awkward silence that everybody needs to realise your talk is over. Note that you can use such a slide as a backup behind your summary slide – if you manage to finish your talk on the summary slide, the audience never gets to see it. If you botch it, you can quickly jump to the summary slide.

In most situations, however, I consider thank you slides and any questions? slides pointless, as they do not tell the audience anything new and are things that you or the chair have to say. A quote would distract the audience from the questions – unless you are going to read it, but then the quote has to really fit the occasion. Your contact information does not need an entire slide and can usually be fitted on the bottom of the summary slide.

Finally note that on some rare occasions, the following order of slides may work:

1. main talk with main results
2. summary and outlook
3. one or two appetiser slides illustrating first steps into what you just announced as future work, e.g., to show that you paved the way for something interesting.

In this case, you can either jump back to the summary slide or stay on your last appetizer slide – depending on what is more attractive.

Another alternative I have tried recently is putting thumbnails of all the previous slides on it:

It's just a recent experiment, though; I don't have enough data to tell if it's the best last slide. Apart from the eye-candy, I think it can be useful as a pseudo-summary if your slides are sufficiently recognizable (for instance, if they have pictures).

For sure it helps solving the problem mentioned in another answer: "You had this formula on one slide. Can you go back? ... No not that one, before that ... Ahh yes that one."

It's kind-of tricky to do automatically in beamer, though. You can always do it manually by copying the output file somewhere else and specifying the page numbers manually, which is how I achieved it:

\begin{frame}
\begin{tabular}{cccc}
\includegraphics[page=1, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=2, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=3, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=4, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} \\

\includegraphics[page=5, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=6, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=16, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=17, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} \\

\includegraphics[page=18, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=19, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=35, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=45, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} \\

\includegraphics[page=46, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=50, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=51, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} &
\includegraphics[page=52, width=0.2\textwidth]{poloni_slides_pdfpages.pdf} \\

\end{tabular}

\begin{block}{}
\centering Thanks for your attention!\\
Questions?
\end{block}
\end{frame}

If I were on your thesis committee I would be most happy with your slides if they were your original work and represented your own personal tastes and sensibilities. Therefore I would be more impressed with even a goofy or weird last slide that I knew you actually came up with yourself and made sense to you, than one that was proposed to you by people on academia.se, even if it were ostensibly more professional looking or slick.

In other words, the "best last slide" is, by definition, whatever you decide it is.

And yes, I realize this is a bit of a smartass answer. I am trying to make a point here about the value of original thought, and hope that some people will find this perspective helpful or thought provoking. But to anyone who doesn't get it or thinks I am barking up the wrong tree, feel free to downvote this answer.