King of Fighters XI (PS2)
What's more hardcore than a 3D beat-'em-up? A 2D beat-'em-up - and they don't get much more hardcore than King of Fighters. The series has traditionally been the opposite of Street Fighter II. The controls are more about timing and the game is less flashy. After some faltering steps into the third dimension (as even SNK Playmore realizes pretty 3Dgrabs more gamers than 2D perfection), it's KoFXI that hits the hardcore nail square on the head.
Here's the deal - if you don't learn how to play it, you will not succeed. You simply can't get by with some lucky button-mashing. By the time you reach the boss, you'd better have your evades and cancels worked out, because his attacks are simply inescapable. Even the most nimble-fingered gamers won't be able to avoid every one of his projectiles through pure skill alone - you need to learn how the game works. And when you start... well, we'll see you in a few months.