"Stellar ECs" are check boxes that are strategically checked at the right times, which create the right perception of one's self by ADCOMs.
In my opinion, the people that unfortunately come off as looking the best are what I call "ZERO to Mother Teresa" applicants. This is an applicant that will go from not doing any ECs to suddenly becoming the next Mother Teresa virtually overnight (when they officially become pre-med). The applicants that come off as bleeding heart pre-meds hell bent on saving the world will ultimately be the ones who gun for derm and rads residencies once they get into medical school. Thus, I feel that the students that "accomplish" most and deemed the most desirable by ADCOMs will turn out to be the complete opposite.
It's all about the dog and pony show, since ultimately, most regular college students aren't going to be doing any of these activities. The dog and pony show must be very carefully timed. It's best started early since commitment somehow equals genuineness. Therefore, someone who starts these activities as a freshman or sophomore might look like an awesome person, while someone who starts these very same activities within a year of applying will look like a selfish box-checker.
Typically, the outline of your typical SDN cookie cutter pre-med is: Clinical volunteering, shadowing, non-clinical volunteering, and research. The first two are a must, unless you are doing certain types of paid clinical work. Otherwise, the last two are bonuses. You need to be able to show ADCOMs that you know what you're getting into, and that you're altruistic. Therefore, shadowing is an obvious box-checking activity, and ADCOMs don't mind if you treat it as such. Volunteering, on the other hand, should be a long-term commitment. I wouldn't cut it anytime before filling out AMCAS at minimum since that will blow your facade and show your true intentions, unless of course you were doing things like this long before you were ever a pre-med.
Altruism is a very rare trait. That's why I'm always in awe, and greatly admire people who give back to the community when they aren't doing it for any other ulterior motive. I can't say that I've seen too many non-pre-meds lining up at hospitals to clean beds or stock linens for free. So a majority of people, who don't do this normally (which of course there is nothing wrong with that), will have to put on a nice show for ADCOMs. The important thing is to not blow your cover. Just keep doing it, and if you really don't care, drop it once you've already interviewed at the school.
Things like research are important for research-heavy schools, and less important for others. Of course it's good to check that box though.
Hobbies and other things are bonuses which can create good talking points.
Ultimately, what do I think are the most stellar ECs? The ones that help the most people, because that's the point after all, right?
Well never once have I ever heard anyone on this site asking how they can help more people. They only ask about how they can help themselves. I don't think I'll find anyone looking to help anyone but themselves in the near future.