Nothing more boring for viewing (sound is another matter) than watching an F1 race track-side. It's a complete rip-off too. Testing gives you much more value, including pit-lane walks usually. I'd go to more if it was a little more local.
I can't stand BTCC in terms of the standard, but we and some guys from a motorsports forum always make a habit of going to a least 2 races. Either Oulton Park or Donny along with Knockhill (best spectator track you'll ever see). We're doing Brands DTM again this year too.
If you're there for the racing rather than the laugh, LMS is an excellent spectacle, with sounds about 8 million times better than any recent F1 car - racing for 5-6hrs.
Think we're just doing Brands DTM and Knockhill BTCC this year though. Keep meaning to attend the Nurburgring 24hrs (proper track!) but something always keeps us.