Pac-Man is at the top of my list of 2600 games that I greatly enjoy, along with Ms Pac-Man. It isn't the same as the arcade game, not even close, but playing on game option 3 with the monsters slowed down I can actually play it for quite a while and get through something like 10 levels before the monsters speed up to a point where it gets difficult. I love Pac-Man. Maybe it's because I'm not necessarily looking for a challenge. I play video games to have fun, not to be a masochist. While Pac-Man is flawed (I won't argue with that), it is genuinely fun to play.
Star Ship is my favorite "short game that lasts two minutes", in the same category as Air-Sea Battle, Canyon Bomber, and other such games. After struggling with StarMaster and a host of other games where the enemy relentlessly fires back, I get to have two minutes of fun shooting Klingon warships, flying saucers, and pink robots. Don't mind me, just cruising along in my invincible star ship! Sure, there are asteroids floating around, but all they can do is slow me down. I'm not trying to get a high score or anything, just having fun... in my invincible star ship where nothing can kill me. Yeah! Eh, who am I kidding? It sucks. But I like it.
Defender was great back in the day. I was just a kid at the time, so what did I know? I played it to death when I was young, along with Pac-Man and many others. Defender hasn't aged well as a video game, in my opinion. The arcade game isn't very playable by current day standards because the controls are so counter-intuitive. I realize there are players who mastered those controls and got it down to an art form, but let's face the truth here: the controls were clumsy. The Atari 2600 fixes that problem with intuitive controls (just point your ship in the direction you want to go).
Superman (also mentioned here as a bad game) is actually kinda fun when you know what's going on and what you have to do. I like it. It's not awesome, but not terrible either.
Empire Strikes Back is good. I like it.
H.E.R.O. I have mixed feelings about. It's good, but you really have to learn the levels by trial and error, memorize your route through each level, and then play it like a proper badass. Not one of my favorites, but not awful either.
Tutankham is one of my favorite games, although it's too hard for me. I still like it, though.
Good to see that no one mentioned my all-time favorite game: Gravitar. I mostly play on game option 5 (no gravity), which is ironic because the game is about gravity. Option 5 makes it a completely different game, harder because you have to thrust everywhere, and easier because you're not struggling against gravity the whole time.