booktabs

Professional table rules (\toprule, \midrule, \bottomrule) for clean, readable tables.

Example
Preamble

$Rendered LaTeX output$

\begin{figure}
  \caption{Simple Table}
  \label{tab:simple}
  \centering
  \begin{tabular}{ll}
    \toprule
    Heading1 & Heading2 \\
    \midrule
    One      & Two      \\
    Thee     & Four     \\
    \bottomrule
  \end{tabular}
\end{figure}

$Rendered LaTeX output$

% Source: https://tex.stackexchange.com/a/468994/9075
\begin{figure}
  \caption{Table with diagonal line}
  \label{tab:diag}
  \begin{center}
    \begin{tabular}{|l|c|c|}
      \hline
      \diagbox[width=10em]{Diag \\Column Head I}{Diag Column\\Head II} & Second & Third \\
      \hline
                                                                       & foo    & bar   \\
      \hline
    \end{tabular}
  \end{center}
\end{figure}

% Nicer tables (\toprule, \midrule, \bottomrule)
\usepackage{booktabs}