I'd say Trek is easily the best known worldwide. I'm not convinced it's 'mainstream' anywhere though.
Where do you draw your line for "mainstream"?
In 1994, Trek was a top 10 show, it was covered on Entertainment Tonight and in magazines like TV Guide constantly that year, it was heading to the movies with much anticipation, packed sports stadiums were airing the final episode of TNG, nerds and non-nerds enjoyed the show (different TV landscape, sure, but that can't be faulted, it was what it was), I think most would agree that the word "mainstream" would have little difficulty being applied.
Right now? Maybe not, the movies did well enough, but Dr. Who still has a lot of catching up to do.
A single international event doesn't compare to what Trek did basically every week for years, and then even bigger with movies, and continues to do now with movies. And will most likely come back with a TV series in one shape or form in the not too distant future. While most people in North America (and probably elsewhere) will still have no idea what a Dr. Who is.