Need some help deciding.

If you're not going for 100%, Arkham games are not that long. Play Asylum first then something else, rest for later :D

Arkham series for sure ;)

what about Metro, Death Stranding or Stalker?

Yakuza

Arkham shouldn't be too overwhelming as long as you dont go after the riddler trophies (but you don't get the real ending in Arkham knight unless you do the riddler trophies so you'll have to slog through that)

My vote goes to the Arkham series.

Arkham series is great, but I would definitely take a break between the games, open world exhaustion is a thing.

Evil Within or Arkham Series, voted Evil Within.

Evil Within can be really hard and janky. There's an unfinished Letsplay on Eurogamer's channel (yt channel but twitch stream recordings) where Aiofe is really struggling with her sanity sometimes.
Evil Within 2 is much more forgiving and logical in it's gameplay loop. Although more open worldy. I'd suggest watching a few episodes of the part 1 LP to see if you want to do that to yourself.
I second the Metro series as suggested above. Great atmosphere, story and characters.
Concerning RE.
It's totally playable doing the remastered/rebooted parts 1-4. I'd probably wait on an eventual updated 5&6.

Hollow Knight is SUPERB, but sometimes it was a bit too difficult for my taste (and best played blind, discovery is a big thing there). Yet I voted for Arkham above, since it is also an amazing series, and it flows better. But, as people said, you might want to take a short break between games, they are very similar in their main mechanics.

Ok, so I decided to go for Hollow Knight and then the Arkham games. Thanks all.

Bump on the funny jigidi! I woulda say that I place my bet on Resident Evil, but you are all set. Nevertheless, I can say that this franchises won't be enough to beat Bioshock... one of the best games we can ever play!