I've heard that admissions offices look most favorably on philanthropic activities aimed at disabled woodland creatures, so you should think about starting a charity for three-legged squirrels...
...or I'm joking, and there probably isn't one activity/organization that impresses highly selective colleges the most. In truth, there are a variety of activities that could impress a college, but what stands out more are the ones which you initiate yourself--a club or organization that you started, for example--and ones in which you took a leadership role. Just being a "member" or an organization doesn't say much about, and might not impress a college, because how much effort does it take to attend a few meetings or be on an email list? Colleges like to see where you stand out in an activity, not merely be a follower.
More importantly, pick an activity/organization (or two) early on--like in 9th/10th--grade--and stick with it/them. Admissions officers can be a bit dubious when a student claims to be participating in 437 different activities/organizations while also maintaining a 4.5 GPA, taking SAT prep classes, etc., because in reality nobody has that kind of time.
Colleges want to know that you're doing something more than parking yourself in front of your Facebook after class ends, but remember that extracurricular activities can include jobs (shows maturity and self-sufficiency, and supports your family!), taking care of younger siblings, being involved in faith-based activities, and a number of other things--up to and including the helping those three-legged squirrels.