Generally, ECs are most valuable if you achieve highly in them. Time spent on an EC without high levels of achievement, or being a “joiner” for lots of ECs, is likely to be less valuable.
For example, if you play a sport and place highly in the state championship, that is a lot better than playing the sport for an equivalent amount of time but not even getting to be that good in your school, or playing several sports at the beginner level.