\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{2}{cc}}
\toprule
\multicolumn{2}{c}{\begin{tabular}{@{}c@{}}A \\ B\end{tabular}} \\
\multicolumn{2}{c}{\textsuperscript{Sup}} \\
\multicolumn{2}{c}{\textsubscript{Sub}} \\
\multicolumn{2}{c}{AB} \\
\multicolumn{2}{c}{Label\tnote{1}} \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft]
\footnotesize
\item[1]Annotation
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}