Well there are three components of fitness (strength, endurance, and cardio), so there isn't going to be a single answer.

But in general, the more boring something is the more effective it is. Just sitting on the rowing machine at 75% of your max HR for 90 min is always going to be more effective than taking a class where you are constantly going at different paces but are doing it more for entertainment rather than because you're trying to do intervals.