It's hard to say. Aside from picking the one that you are the proudest of, which experience will make the most sense/ flows the best along with your application? Even if topic is the "best", if it doesn't flow well with your other aspects, then it will stick out like a sore thumb (in the negative way).
At first glance, it seemed that someone other than yourself was achieving something and you were only a conduit. But, thinking about it a bit more, it seems that two of your proudest acheivements have been instances where you have helped others in substantial ways. Go with either of those and focus on why helping others achieve academic success or mental health makes you proud to have been involved.