Errr, depends on what strategy you're going to take. If you do a game like Elden Ring, there's going to be a LOT of potential viewers but there's also going to be a LOT of competition for them. If you pick out some niche game like X4:Foundations or Troubleshooter: Abandoned Children, then there's little (if any) competition but there aren't that many people that are going to be interested, either.
If you haven't done ANY videos yet, just pick whatever game. You need to figure out what you're doing first.
Dying Light *looks around* Phew did not get banned
It's difficult to say for certain without knowing anything about you. I can say this though: People like different things and there are a lot of niche YouTubers out there earning just enough to be relaxed, which is often how the larger YouTubers start out there before building up their audience while doing what they love. It's extremely hard to get popular though, so If you want fame and fortune from the get-go, you might want to look elsewhere. Hundreds of videos, maybe even a thousand it could take before you actually get a steady stream of audience and income from your job IF you are lucky. I wish you good luck and have fun gaming which should always be first priority!
The key is to play games you like and not just the most popular new games. You may initially get more viewers from titles that are new and popular like Elden Ring but if you're not into a game, why put yourself in that position. Just play what you like and be on frequently and if people like what they see, you won't have a problem gaining popularity while enjoying games. A lot of streamers I talk to seem to forget how to have fun because they only worry about views rather than playing games they actually care about
For a retro game, yes that's an excellent suggestion!
People often forget to start small and then try and make their way up to the big players. A lot of people don't even bother understanding that their system(s) has(have) a finite amount of oomph it can deliver before the game and eventually the stream/video conks out.