RocketTheme are one of the oldest and most popular Joomla template developers.
I recommend RocketTheme templates as this is what I use on nearly every website that I build (including this one).
RocketTheme templates are based on the open source Gantry Framework and RocketTheme provide a few free templates in addition to their paid templates.
Support via their forums is excellent.
RocketTheme are diligent in retrospectively upgrading their older templates as new versions of Joomla have been released with the result that websites with RocketTheme templates are often the simplest to migrate to a new version of Joomla.