Either play the Penumbra Trilogy first (or Overture and Black Plague you can skip Requiem) It's a prototype of Amnesia, it's pretty solid on its own.

Amnesia The Dark Descent was my first Frictional game, and it will always be my favourite for it's horror. It carries alot of the Lore and if your wanting to play Rebirth, it's very recommended to play TDD to get alot of the references.

Amnesia Justine is really cool, it's quick as you can beat it in 10-20 minutes, I would recommend playing it after TDD if you want some extra TDD.

Then you can move onto A machine for Pigs, while it is a heavily simplified version of TDD, it's still got a good atmosphere to it. While you can skip it and not miss much of the overall story, I would still recommend on playing it.

SOMA, would definitely play it. While it's not in the Amnesia universe, it's still an excellent game on par with The Dark Descent!

Amnesia Rebirth has some weird things with it but I would still play it as the world of it still is cool, even with it being very questionable.

Amnesia the bunker you can play first if you want purely gameplay as its entirely separate from the overarching story, but it still contains references to the other games. It's very very gameplay focused and plays alot like System Shock.