Inside the home directory there are very little recommendations, about what to put where. In this particular case it's completely up to you. I would discourage you to put it in /opt.
The only 'standardized' folders of my knowing inside /home/$USER are .local .config and .cache. Like all hidden folders in /home/$USER - those with a dot infront - they hold user specific application settings and files. They can be shown in nautilus by pressing Ctr-h.
I would recommend to make the TorBrowser folder also an hidden one by editing it after unwrapping. After the first start of Tor Browser Bundle, dump a copy of the Start TorBrowser file to ~/local/share/applications/ this will allow you to find the TorBrowser in your application menu (unity,gnomeshell-activity, etc) and if running will display it as TorBrowser and not Firefox. Inside a terminal this can be done, after moving inside the TorBrowser folder via cp start-tor-browser.desktop ~/.local/share/applications/start-tor-browser.desktop.
As TorBrowser saves it's downloads inside it's own folder I made a bookmark inside nautilus called Townloads, for easier access.