Games a would reccomend would be adventure games, which you can do a series on. Survival games, for more series. Funny/short ones with friends that leaving people hungry for the next upload.
Portal/Portal 2 are both great puzzle games, and people love watching you get stuck on a puzzle you have already solved, though co-op portal and custom maps are what really make it a blast.
Survival games can range from Do Not Starve/and the co-op one, 7 Days 2 die, Ark (would not really reccomended... due to the large amount of these videos already being produced)
As for adventure games, Half Life would indeed be an interesting vid, but games like left 4 dead/left 4 dead 2 are also great (tip: add-ons and custom maps with friends will make it amazing)
Minecraft.... enough said... just mini-games, games, I dont really have an opinion on survival series... do them if ya feel like it.
Garrys mod, the king of many games. Infinity amounts of games at your finger tips, Though wait Play with just friends would be more reccomended due to squeakers, young thugs (kids who think there gansta) trolls, annoying children, ect. So playing with friends (3-6) Due to mic spam, you dont want a large amount.
Games Listed
Portal/Portal 2
Do Not Starve
7 days to die
ark
left 4 dead/left 4 dead 2
half life
minecraft
garrys-mod
Hope it helped