What is the best Math Font for use with Minion Pro font?

I suggest to use the MinionPro package. It works under XeLaTeX and pdfLaTeX. It is part of the Font Pro package available from https://github.com/sebschub/FontPro. Follow the instructions there to install it.

The following MWE

\documentclass{article}

\usepackage{MinionPro}

\begin{document}
This is the standard Minion Pro Font\ldots My issue is with the 'f' in italic math mode. It needs more space. For example:

\begin{equation}
f(x) = x^2 + f(n-1) 
\end{equation}

\end{document}

yields

Another solution is to use the MnSymbols package. To avoid the problem with the \mathdollar already defined error one can use the following workaround.

\usepackage{fontspec}
\usepackage{etoolbox}

\defaultfontfeatures{Mapping=tex}
\setmainfont{Minion Pro}

\usepackage{MnSymbol}

% the next line makes the definition of \mathdollar from MnSymbol void.
\undef\mathdollar 

% the next 3 lines reinstate the definition of \mathdollar from MnSymbols 
% at the begin  of the document
\makeatletter
\AtBeginDocument{\DeclareRobustCommand{\mathdollar{\Mn@Text@With@MathVersion{\textdollar}}}
\makeatother

André Miede, the author of classicthesis - which uses Minion Pro, recommends the Euler Math font to use with the latter one:

"[...] loads the awesome Euler fonts for math." [emphesis mine]

(from the classicthesis documentation, page 6)

According to the German Wikipedia article there is a Minion Math font by Typoma, specifically designed for Minion Pro.

Find more information on their homepage at http://www.typoma.com/de/schriften.html