If I type
\boldmath $\widetilde T$\unboldmath
I get
where unfortunately the tilde above the T is a little flimsy for my taste.
Is there a better way to do this?
I found a paper that has something, that appears a little bolder, in it. 
[Tex/LaTex] How to get a bold wide tilde
boldmath-modetilde
Related Solutions
\documentclass{minimal}
\usepackage{listings}
\lstset{
literate={~} {$\sim$}{1}
}
\begin{document}
\begin{lstlisting}
awk 'a !~ $0; { a = $0 }'
\end{lstlisting}
Variables in Awk don't need to be initialized or declared before they are being used.
\end{document}
I suggest you use the listings package as follows:
\documentclass{article}
\usepackage[formats]{listings}
\lstdefineformat{R}{~=\( \sim \)}
\lstset{basicstyle=\ttfamily,format=R}
\begin{document}
\begin{lstlisting}
lm.1 <- lm(y ~ x1 + x2)
\end{lstlisting}
\end{document}
The option formats provides a mechanism to substitute certain characters by other code. Here we set up an R format where ~ is replaced by \sim in math mode. Using lstset this style is set for all listings; you could instead add these options to individual code listings or define a specific style via lstdefinestyle{myR}{basicstyle=\ttfamily,format=R} to be used as \begin{listing}[style=myR] ... \end{listing}.
Best Answer
I'm afraid you hit on something that can't be easily solved. Here for comparison all possible solutions for making math stuff bold and then a handmade bold tilde T:
Using .8em in the raisebox will be about the same height as the other tildes but I personally liked it better this way. I'm not saying it's pretty and I'm not saying it's easy :/