5. River Raid
An early vertical-scrolling shooter, River Raid had a simple premise: fly a plane up an endless river, shooting bridges and enemy craft as you go. The plane, however, couldn't fly over land (an article in Creative Computing: Video & Arcade Games helpfully explained this was because you were flying low to avoid enemy radar). You could only have one missile in the air at a time, meaning you could fire faster if you hit enemies close, and misses were punished by forcing you to wait till the shot vanished off the screen as in Space Invaders (the same article linked above has this as as a budget-saving feature of your plane to be discussed with Congress in the next fiscal year). Fuel was limited, and though there were tanks on the water that could be flown over to refuel they can also be shot for points. Occasionally the river split around an island, another split-second decision.
These little complications made River Raid a challenge, but what made it fun was momentum. The plane's speed could be modified with the joystick, but there was no way to stop – you were always pressing onward, up the endless River Of No Return. As you did it changed, enemies displaying new movement patterns and fuel tanks becoming rarer. There was a sense of taking a journey that was rare in games of the 1980s, that your main motivation wasn't just making a score go up but seeing what was beyond the next bridge.