The best ECs are the ones you like. It is always good to find some that contribute to the community. Or the best may be getting a job or babysitting your siblings if necessary.
It is good to do some in your area of interest if applicable and some that let you explore new areas.
Check out “How to be a High School Superstar” by Cal Newport.
“The basic message of the book is this: Don’t wear yourself out taking as many classes as you can and being involved in every club and sport. Instead, leave yourself enough free time to explore your interests. Cultivate one interest and make it into something special that will make you stand out among the other applicants and get you into the toughest schools, even if your grades and scores aren’t stellar. Newport calls this the “relaxed superstar approach,” and he shows you how to really do this, breaking the process down into three principles, explained and illustrated with real life examples of students who got into top schools: (1) underscheduling—making sure you have copious amounts of free time to pursue interesting things, (2) focusing on one or two pursuits instead of trying to be a “jack of all trades,” and (3) innovation—developing an interesting and important activity or project in your area of interest. This fruit yielded by this strategy, an interesting life and real, meaningful achievements, is sure to help not only with college admissions, but getting a job, starting a business, or whatever your goals.”
http://www.examiner.com/review/be-a-relaxed-high-school-superstar