Make space between rows constant

Perhaps a combination of things are needed here:

• Reducing the height of the odd-one-out formula using \smash; this will actually make it have a height/depth of 0pt.

• Substituting the odd-one-out height for something else using \vphantom{<stuff>}; this will not print <stuff>, but instead insert a box of zero width with height/depth of <stuff>... like a vertical strut.

• Adjusting itemsep to make the visual appearance acceptable; enumitem works easiest for this.

\documentclass{article}

\usepackage{enumitem}

\begin{document}

\noindent
\begin{minipage}[t]{.3\linewidth}
\begin{itemize}
  \item $\displaystyle \lim_{x\to x_0}f(x)$
  \item $\displaystyle \lim_{x\to x_0}f(x)$
  \item $\displaystyle \sum_{i=1}^N i^2$
  \item $\displaystyle \lim_{x\to x_0}f(x)$
\end{itemize}
\end{minipage}
\hfill
\begin{minipage}[t]{.3\linewidth}
\begin{itemize}
  \item $\displaystyle \lim_{x\to x_0}f(x)$
  \item $\displaystyle \lim_{x\to x_0}f(x)$
  \item $\displaystyle \vphantom{\lim_{x_0}}\smash{\sum_{i=1}^N i^2}$
  \item $\displaystyle \lim_{x\to x_0}f(x)$
\end{itemize}
\end{minipage}
\hfill
\begin{minipage}[t]{.3\linewidth}
\begin{itemize}[itemsep=2ex]
  \item $\displaystyle \lim_{x\to x_0}f(x)$
  \item $\displaystyle \lim_{x\to x_0}f(x)$
  \item $\displaystyle \vphantom{\lim_{x_0}}\smash{\sum_{i=1}^N i^2}$
  \item $\displaystyle \lim_{x\to x_0}f(x)$
\end{itemize}
\end{minipage}

\end{document}

The left list is an original, unadjusted list. The middle list uses \smash to remove the height/depth of the \sum but also inserts a \vphantom of the tallest item in the remainder of the list (in this case, \displaystyle\lim_{x_0}). The right list adds some vertical space between items by setting itemsep=2ex.