According to the Atari 2600 FAQ, at http://www.atariage.com/2600/faq/index.html?SystemID=2600 , a poll on rec.games.video in 1992 determined that the five most-loved Atari 2600 games were:
1. Adventure
2.(tie) Solaris, Yar's Revenge
4. Asteroids
5. River Raid
Presumably, 'Pitfall!' is a very close sixth. As for the most popular cartridges, it's likely that 'Combat' (which was given away free with early systems) tops the list, with 'Asteroids', 'Bezerk', 'Space Invaders' and 'Defender' following closely behind.
At the bottom of the list would be Atari's notoriously rushed and awful conversion of 'Pac-Man', a commercial success but a terrible advertisement for the company, and the infamous 'E.T. The Extra-Terrestrial', which was such a disaster that literally millions of unsold cartridges were simply dumped as landfill, a cheaper option than shipping them to stores which did not want to stock the game in any case.