For my money, in terms of accessibility and fun while having a steep but manageable difficulty curve - the 2600 game that stands up the best today is Kaboom. It can induce an almost zen-like state that is very hard to capture in gaming.
Fully agree. I would love to see a modern version (complete with control wheel) in HD with smooth frame rates. Still have found memories of sitting in front of an RCA XL-100 console tv with my eyes burning trying to catch all those bombs. Wish I would have sent in my score back in the day to get the Patch from Activision.
River Raid was similar, but Kaboom was nice because it used the knobs.