Cycling's the main one for me, but I also follow lots of other sports with enthusiasm. A lot of the more mainstream sports that I follow I don't comment on because the forums are often filled with partisan bile spewed by otherwise quite clever people. Quite a few of the more niche sports I like don't really have vibrant forum communities that are active enough to merit hundreds of posts. That's one advantage cycling has, the cast of thousands, it means there's always something going on. I used to use some motorsports forums, talking primarily about endurance sportscar racing, but nowadays I tend not to.
(oh, and in answer to your question about Ángel Madrazos of car racing, I nominate Estebán Gutiérrez, a young Mexican racer. He's 19, he won the Formula BMW series in '08 - he and Tiago Geronimi put on a better race around the Valencia circuit than the F1 cars managed anywhere all year - was the top rookie in F3 Euroseries in '09 and then dominated the GP3 series in 2010. He was 4th fastest at the Rookie Driver F1 test in November despite being in the Sauber, and is now their reserve driver, as well as doing GP2 Asia. I used to follow the sport much more closely than I do now, but I still keep my eyes peeled for the results of guys and girls I like)