2. Kill Switch
The tag-line of this one is, 'Take Cover, Take Aim, Take Over', which should give more than a subtle hint of the main gameplay mechanic here. If Gears Of War would later perfect the art of cover-shooters, Kill Switch helped pioneer what is now taken for granted. Playing through instantly feels comfortable, due to how many games feature the ability to snap to cover, peek out and shoot, and even though Kill Switch doesn't have a great story, doesn't feature eye-popping graphics (even for the PS2), or have much replay value, it's still worth checking out. If you like to see how far various mechanics have come in gaming, the best way is to go back to the start.