I would have to say that games that are worth owning for the 2600 that I don't believe have been stated yet include the following
Joystick games
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Millipede, pengo, thrust+, human cannonball, sky diver, entombed, adventure, jammed, crystal castles, bump and jump, saboteur
Paddle games
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Warlords, marble craze, super breakout, canyon bomber, video olympics, street racer
Keyboard controller
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Brain games, star raiders
There are many other great games, but these are the ones that come directly to mind.
-mike