It seems that "unpopular" and "underrated" are closely associated in people's minds, but I didn't immediately make that connection when I read this thread title. I made the assumption that the TS was talking about games that other people had actually played but decided they did not like. Then of course you have to hear about the fact that they don't like the game, play it yourself and decide you like it. Anyway blah blah
Some that were mentioned that I also like are Star Ship and Communist Mutants from Outer Space. I seem to read negative things about these two from time to time.
One game that I haven't seen mentioned in this thread is Mouse Trap. Every once in a while Mouse Trap seems to come up and people aren't very favorable toward it. I like it and like the fact that holding down the button has a different function than just tapping it. How many other 2600 games have that same mechanic?
I just recently discovered Commie Mutants. I think it's a lot of fun. Maybe not the greatest Atari space shooter but certainly not bad IMO.