AtariAge is a nice resource for collectible Atari game cartridges. They have a rarity guide that is a general indicator of how easy or difficult it is to come across a game.

Their description of collecting for the Atari 2600:
“The Atari 2600 is the system most people collect games for. This is likely due to more people owning a 2600 than any other classic gaming system. Back in its heyday, who didn’t have an Atari? This has two effects. First many people have a nostalgia for the 2600 and pick one up to play all the games they remember from their past. And second, because so many people had one, it’s one of the easiest systems to collect for as there are so many games floating around.”

The titles in the unbelievably rare (10) category:
– Air Raid from MenAvision
– Alien from Xante
– Atlantis II from Imagic Picture
– Beany Bopper from Xante
– BMX Airmaster from Atari
– Copy Cart from VidCo
– Crypts of Chaos from Xante
– ECPC Cartridge from Romox
– Eli’s Ladder from Simage
– Gauntlet from Answer
– Karate from Ultravision
– Magicard from CommaVid
– Mangia from Spectravision
– Motorodeo from Atari
– Music Machine from Sparrow
– No Escape! from Xante
– Obelix from Atari
– Pepsi Invaders from Atari
– Solar Storm from Xante
– Video Life from CommaVid

Visit AtariAge here >>