So I'll start. Me personally I love watching GP2. But in GP2, interestingly, I prefer to watch the races, while in F1, my focus lies more on the championship tables. A close championship fight seems more enjoyable to me than a close fight on track, whereas it's the opposite for GP2 for me. I also love watching the DTM, as I do love watching people race sportcars (albeit a bit more purpose-built for racing) and I do love the great close racing
A friend of mine recommended watching superleague, and Indy too.
Having said that, everything else seems to be slow when you've watched F1
