Core Set for the Arkham Horror The Card Game LCG
You technically only need one core set to play but it's recommended you buy two to have more deck building options. If you're on the fence, buy one to try the game out and then buy another one with expansions if you like it.
This game is best played with two people, it can be played with up to 4 but that does slow down the game a lot. If you play it alone I recommend you play it "two handed", controlling two investigators essentially playing on your own as if there were two players.