There's tons of flash games out there, including horror, that haven't been covered yet, you could make a potential series from them. Google is your friend!

Try playing Hello kitty adventures. Scariest shit ever. But seriously , there are plenty of games you could play.
Google some, and please you're still in that stage when it doesn't matter what your viewers want you to play (since there aren't many ) so pick up one
that you like.

There's flash games like The House 1/2 and Exmortis 1/2 that are good and properly scary, it's just a matter of looking hard enough to find the good ones! Try Ghostscape, The Halloween and Ask The Spirits 2 (this can also be hilarious). They're all sort of basic point and click games and can be fairly quick to play through!

In my opinion, you should play Half-Life 2, it has plenty of scary mods you can play and most of them are a lot of fun. Another game I suggest you to play is the Slender man games, they can be pretty scary sometimes and it's really fun to see people play it, so I'm sure your fans will enjoy it. Good luck, and keep up the good work

I bet you 90% of the people who give you suggestions will say Minecraft.

How about Silent Hill?

Well, I think the horror genre is the most popular in let's plays, so it's kind of hard to suggest something that hasn't been tried out from that genre : P

I'd say go with Exmortis. I personally have never played horror games, but from what I've heard and seen, those two games are pretty scary. But then again, you may be stuck playing the same games that everyone else has played. Jump scares are good, and in some cases funny for the viewer, but you don't want that to be the only reason you play for others (I think). There's also Ib, which doesn't have a lot of "in your face" scares. And that's pretty much all I can think of right off the top of my head. Pretty lame, huh?

kongregate.com

Lots of games.

Check this site out: moddb.com/games it has a search filter so you can find games of different genres and developers.