H

The H produced by the \mathscr macro of the MathTime Pro 2 Professional font package -- note that it's not free of charge -- may (or may not...) be what you're looking for:

\documentclass{article}
\usepackage[mtpscr]{mtpro2} % http://www.pctex.com/mtpro2.html
\begin{document}
$\mathscr{H}$
\end{document}

Many more possibilities present themselves if you're willing and able to use LuaLaTeX and/or XeLaTeX and the unicode-math package, as several modern math Opentype fonts feature a math script alphabet. Here are four more possibilities; the last one (from TeX Gyre Pagella Math) may be of particular interest to you:

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{unicode-math}
\setmathfont[version=XITS]{XITS Math}
\setmathfont[version=Asana]{Asana Math}
\setmathfont[version=Pagella]{TeX Gyre Pagella Math}
\setmathfont[version=Termes]{TeX Gyre Termes Math}
\begin{document}
\mathversion{XITS}    $\mathscr{H}$ \quad
\mathversion{Asana}   $\mathscr{H}$ \quad
\mathversion{Termes}  $\mathscr{H}$ \quad
\mathversion{Pagella} $\mathscr{H}$ \quad
\end{document}