Wow, but this is a difficult question. It depends on what you mean by "motorsport". What are you counting? Technology? Speed? On-track action? Driving quality? Money?
The last one is easiest. As this week's published financial results show, F1 is still the most costly and yet most remunerative racing class in the World. Also unquestionable is the issue of speed. Last year's pole lap of Silverstone was 1:29.8. Pole for the LMS 1000km race was 1:43.5. That's a gap of nearly 14 seconds, meaning that the LMP1 cars were over 15% slower around that fairly high speed circuit. Just absorb that for a second. Even F1's closest analogue, GP2, had a pole time of only 1:39.2. Despite Max Mosely's carping, this would not have been fast enough for a spot on the F1 grid. Both the HRT cars were in the 1:36s, and the then-hypothetical 107% time was 1:36.1. F1 cars are in a class of their own, literally and metaphorically.
Arguably, the quality of the drivers is also unsurpassed, although their subsequent performances in other series when they leave F1 suggest that this isn't as clear-cut as Bernie (and those drivers' agents) would have us believe.
And so on to the other end of the scale. F1 definitely does not lead the World in terms of on-track action. The sheer spectacle and experience of having a pack of F1 cars screaming through the first lap of a GP is absolutely unique, but until the last couple of years it did require a certain fanatacism/masochism to keep 100% focused throughout the race. Personally, I also don't like the rough and tumble of touring cars, but the precision passing and rapidfire tactical assessments made in the lesser open-wheel formulae are something else. Personal opinion only, I guess, but what's this place for if not an opinion or two!
As for technology, well, that's moot too. The regulations are far too restrictive, of course, but what they do have is at the absolute peak. I'd like to see alternative fuels properly explored, and a return to body-generated downforce, but there are still technologies being introduced on F1 cars that are filtering down to road vehicles. Fiat's new two-cylinder engine for the 500 borrows an awful lot that Ferrari have learned about pushing the limits of engine design, for instance.