Personally, I would close this for being opinion based, but, since I can't...
You can find different people's opinions all over the Internet (various forums that come up when you Google this question in Hebrew), however, I will try to extract the useful information (and Rabbinic opinions) from the various results...
From one Otzar Hachochma forum, a useful piece of information is Sye510's point above in the comments to Ezra's answer (which also seems to be a popular starter from my non-Chabad experience):
שמעתי פעם שהעניין שאמרו ללמוד בבא מציעא ואלו מציאות תחילה זה לחנכו בזהירות ממון מתחילתו.
More important is the quote there from Sefer Chovas Hatalmidim that children should (and/or do) begin with Perek Shenayim Ochzin, there is a copy of the page in that thread.
Also there, a suggestion from Kuntres Devar Malchus to begin with Maseches Megillah.
Another linked Otzar Hachochma discussion brings various sources in Rishonim about Perek Ein Omdin being the easiest/simplest, and features other interesting sources as well
Here, Harav Yitzchak Ben Yosef (I don't think there is any relation to Harav Yitzchak Yosef) suggests Megillah and Berachos:
ללמוד גמרא עם רש"י זה מצוין עדיף להתמקד במסכת אחת ולהמשיך עם אחרת.
מסכתות בסדר מועד ,טוב להתחיל בלימוד , מסכת מגילה ברכות.
אבל ללמוד מסכת אחת.
The "Tzevet" of Yeshivas Tzefas here suggests Megillah/Taanis/Chagigah for a child beginning on his own.
Rav Yosef Alnakwa (?) here suggests the first Perek of Berachos for an older beginner.
(I would also check out this link, but the video doesn't load for me.)