The problem is that streaming is very different to making YouTube videos - with YT vids, people tend to want shorter entertainment fixes, whereas withas with streams people join to watch longer so it's better for longer let's plays.
Basically it comes down to YouTube being something of a minefield, where you'd looking for something that everyone is wanting to watch but not so many people are making videos on. For the most part it's not possible, especially when it comes to the most well known YT creators as they'll have early access, etc.
It's not impossible though. Don't let what I've just said discourage you. I know someone as a close, personal friend to have played a game that everyone is playing and just happened to have caught the edge of people's attention and grown massively from that. They are now 25k subs or more larger than when I first knew them. All you need to do is going for games that you feel are both going to be very popular and ones that you enjoy - one of the biggest things is that people watching will be able to easily tell if you're having fun with a game or now, and they'll respond in kind.
Not sure how much this will help but this is basically that best kind of advice I've ever received, and all I've ever felt best to pass on, so hopefully it's helpful.
If people were able to determine which games would be most popular on YouTube, that people are not already playing, then it'd be easy and everyone would be doing it.
The thing about YouTube is that videos and certain topics go viral randomly. You really can't predict the popularity of a video or game in general on YouTube. Of course if you have a large audience you can pretty much get views on anything.
However, a small channel is just an experimental program. Constantly creating new content, changing things, playing different things and uploading a various of different material to see which performs best.
It might not be the game in general. The titles of your videos could effect the traffic towards the video as well. You could have a game that's going to go mainstream, but with the wrong title you could miss the opportunity.
I don't have the secret formula for you though. Its a game itself, YouTube. You play with different videos and titles, and keep improving your content. The content is what makes people come, stay and continue coming back. If the content is not good enough to entertain or teach the person who is searching for content related to your topic, then they won't return to the next video. You have to figure out how to keep those people coming back.
I know I got a little off topic here, but there's really no way to tell what the best games are for small YouTubers. Only advice I can give is to watch popular YouTubers and produce better content on games related to what people are currently watching.
If people are watching that type of content, you have the opportunity to take some of that viewership with your own content because lots of people should be searching for content around that game in particular. So in a way, the way to find out the best game for small YouTubers is to see what popular YouTubers are already playing and getting lots of views on.
New Indie games. Most the time they are free so you dont spend money and you might be the first to play it if you upload soon after it's released.
Also, YouTube likes to bury small channels so not only are you competing against all the other gaming channels for ranking, your also competing against unrelated things like cat videos until you gain momentum. Best way to get views is sharing on communities. Indie communities love watching gameplay. It saves them from having to play everything.