Elegant fractions in one line

It is nearly always best to use the slashed form for fractions in the text:

\documentclass{article}

\begin{document}
\( x_F \equiv p_z^{}/p^{\mathrm{MAX}}_z = p_z/(\sqrt{s}/2) = 2p_z/\sqrt s \)
\end{document}

Note that I have added brackets to the second fraction.

There is also nicefrac package which provides \nicefrac{<Nr>}{<Dr>}:

\documentclass{article}
\usepackage{nicefrac}

\begin{document}
\( x_F \equiv \nicefrac{p_z}{p^{\mathrm{MAX}}_z} = \nicefrac{p_z}{(\nicefrac{\sqrt{s}}{2})} = \nicefrac{2p_z}{\sqrt s} \)

\vspace{.5cm}        %% Andrew's method for comparison purpose.
\( x_F \equiv p_z^{}/p^{\mathrm{MAX}}_z = p_z/(\sqrt{s}/2) = 2p_z/\sqrt s \)
\end{document}