Keystone Kapers
Pitfall 1 & 2
River Raid
Kaboom
Adventure
Yar's Revenge
Warlords
Space Invaders
are all solid choices.
Do not get E.T.
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Baltimore, MD
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Brunswick, GA USA
I think someone said Joust but I'll repeat it because it doesn't get enough attention. Moon Patrol was okay. Frogger was good.
Find Solaris if you can. It defined the limits of what a 2600 can do.
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Venezuela
this are some of the game i have and i like 
Frogs and Flies
Fishing Derby
StarWars: The Empire Strikes Back
Crystal Castles
Hey! Stop!
Berzerk
Pitfall
Congo Bongo
and many others, but i just remember those
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New York City
My favorites:
Keystone Kapers
Beamrider
River Raid
Totally fantastic games, I can play those 3 for hours on end.
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Brazil
Nobody said Megamania????
And Yars Revenge, of course, I must say again. Best game.
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Brazil
I like to play megamania instead of space invaders and I think it's better! 