Add bold enumerate items on highest level only

This can be accomplished for multiple lists by setting the style for each level of the enumerate environment using:

\setlist[enumerate,<level>]{<format>}

By also setting the before formatter, you can set the text of the entire item in a certain font, as you seemed to desire from your question. Resetting for level two will set the fonts for the label and item back to normal for future levels.

An example:

\documentclass[]{article}
\usepackage{enumitem}
\begin{document}

% Change format of top-level list items
\setlist[enumerate,1]{label*=\arabic*,font=\bfseries,before=\bfseries}
% Reset formatting for subsequent levels; label type makes 1.1, legal-style labels
\setlist[enumerate,2]{label*=.\arabic*,font=\normalfont,before=\normalfont}

\begin{enumerate}
\item One
\item Two
\begin{enumerate}
\item Three
\item Four
\end{enumerate}
\item Five
\end{enumerate}

\end{document}

This gives:

You didn't provide a MWE, but I assume that you are using something like

\begin{enumerate}[label=\textbf{\arabic*}] 
  \item first
  \item second
  \item third
    \begin{enumerate}[label*=.\arabic*]

which does make the second level enumerate items bold. This is not surprising, as you are forcing the label to be bold using label=\textbf{\arabic*}.

Instead, you should specify the font face separately, using

\begin{enumerate}[label=\arabic*,font=\bfseries] 
  \item first
  \item second
  \item third
    \begin{enumerate}[label*=.\arabic*]
 ....

Here's a complete MWE to play with.

% arara: pdflatex
\documentclass{report}
\usepackage{enumitem}
\begin{document}
\begin{enumerate}[label=\arabic*,font=\bfseries] 
  \item first
  \item second
  \item third
    \begin{enumerate}[label*=.\arabic*]
      \item one
      \item two
      \item three
    \end{enumerate}
  \end{enumerate}
\end{document}