\Phi looks bad in Beamer

Rather that import a different \Phi, which will in general not be compatible in style with the pre-existing font, I instead here choose to add small serifs to the existing rendering of \Phi, calling it \altPhi. I have made it work in all math styles.

\documentclass{beamer}
\usepackage{stackengine,scalerel}
\newcommand\altPhi{\ThisStyle{\ensurestackMath{
  \stackengine{-.6\LMpt}{%
  \stackengine{-.7\LMpt}{\SavedStyle\Phi}{\rule{.5\LMex}{.7\LMpt}\kern.01ex}
    {U}{c}{F}{F}{S}}%
  {\rule{.5\LMex}{.7\LMpt}\kern.01ex}{O}{c}{F}{F}{S}}}%
}
\begin{document}
\begin{frame}
$\Phi \altPhi$
$\scriptstyle\Phi \altPhi$
$\scriptscriptstyle\Phi \altPhi$
\end{frame}
\end{document}

With the professionalfonts class option, you can load the font of your choice. Your options include:

• Any OpenType font with unicode-math. You can even add the \Phi symbol from another font with \setmathfont[range=\mupPhi]{SomeFont.otf} (If you select math-style=ISO, which gives you slanted uppercase Greek letters, you would override \mitPhi instead of \mupPhi.)
• Any OML-encoded font with isomath
• Any LGR-encoded font with mathastext
• Any math font package
• Any OT1-encoded font, with \DeclareMathAlphabet. (These have uppercase, but not lowercase, Greek letters.)

If you can choose which compiler and packages to use, I would recommend that you use unicode-math with a sans-serif font such as Fira Math, and compile with LuaLaTeX.

You can switch using mathastext package with the options italic and symbolgreek: but the Greek symbol are in upright mode.

\documentclass[12pt]{beamer}

\usepackage[italic,symbolgreek]{mathastext}

\begin{document}
\begin{frame}
\[\Phi_n(x)=\prod_{\zeta \text{ primitive }}{(x-\zeta)}\]
\end{frame}
\end{document}