4. Defender II/Stargate
The Atari 2600's popularity was built on ports of arcade games. Missile Command, Phoenix, Ms. Pac-Man, Centipede, Pole Position, Q*bert, Moon Patrol – plenty of games successfully made the transition from coin-op to couch. Others, not so much. Though the Atari version of Pac-Man is infamous for its limitations, Defender was an even worse port, the spaceship disappearing every time you fired. Defender II (also released as Stargate) made things right.
In Defender II aliens came in bewildering varieties. There were Landers who abducted humans from the planet's surface and turned into speedy Mutants if they got to the top of the screen, forcing you into a panicky shoot and swoop maneuver to blast them and catch the captive (or just shoot them, if you were a jerk). Blobs were slow and aimless, but burst into Swarmers that zipped directly at you when shot. The wonderfully named Yllabian Space Guppies swam through the sky in pulses and once every 10 waves increased in numbers for an Yllabian Dogfight level.
There was variety in your arsenal as well. The hovering stargate teleported you directly to the location of an abduction, or halfway across the level if no one was in danger. There were bombs, hyperspace, and invisibility, but since the classic Atari joystick only had one button they were creatively placed on the second joystick, occupying its button as well as directional pushes. The scramble for that spare joystick when trouble hit was always a rush.