Hands down, the league I do best in, year in and year out, is the NBA. It's not even close.
I've been a baseball fan for all of my over-quarter-of-a-century years, to the exclusion of all other sports. I never ever watched basketball, hockey, football, lacrosse or anything else...except baseball. I lived, breathed, ate and drank baseball throughout my childhood.
THen gambling started. And I learned that despite my obsession with the game, my poring over endless statistics etc., I could not make money betting it. My overall baseball losses are too staggering to even think about. But in the NBA, a league playing a game I didn't even know the rules to until a couple of years ago, I've always fared much better.
The greater success had to do with one thing: understanding home team advantage. Once I learned the strengths and weaknesses of particular teams, I would invariably look at lines on games and find them skewed -- visiting teams seemed to be treated very generously. When I first started betting, I went pretty heavy on visiting teams, and lost consistently. I then learned that it ain't just about the relative strengths and weaknesses of the competing teams: home court makes all the difference in the world. Since then, I've essentially used one theory when betting the NBA: always take the home team unless there is VERY compelling reason to take the visitors. This doesn't mean I take the home side 90 or 80 or even 70% of the time. All it means is that when I begin my analysis and research, I start with the assumption that I'm going to take the home team, and only if I find strong reason to take the visitor will I switch to the other side.
Seems to work for me.