I’ve been streaming Just Cause 3 lately and it’s been getting a good response. Just Cause 4 is about to come out, so either start with 3, or 4 when it comes out. If you are wanting to really get out of the gate, you could potentially come up with a really unique JC3 challenge or funny situation and make a few 5 minute videos doing challenges and such on those. That’d prime the pump for you in the search results if you plan on jumping into JC4.
Other than that, obviously Fortnite is big, Minecraft still has a cult following, and you can always check out what big YouTubers are playing and just ride their coattales for a bit to gather steam and figure out your style.