Count out most arcade ports - they are going to suck, but the games with really simple premises can be fun.. Games like Frogs and Flies/Air-Sea Battle can be fun with 2 Players Try to get a 7800, if you're interested. Full 2600 BC with the additional option of more games and higher quality arcade ports. It's 8-bit, but it doesn't quite come across that way. It's more of a higher-spec pre-NES pre-1983 Crash style system that came out too late and got stuck in a time warp. (Also, Sega Genesis controllers work great with 2600/7800 in the event that maintaining Atari Joysticks gets annoying)
It's gotta be nostalgia, right? I mean, I don't really like the NES because I grew up with a Genesis first. That's literally the only way I can play the 2600 tbh.
I had a Master System before I had an NES, and I prefer the NES. I just don't get the Speccy love. All those shitty arcade ports....
Battlezone, Demon Attack, Pitfall (1 & 2), Kaboom, Solaris, Star Raiders, Keystone Kapers, Star Wars: The Empire Strikes Back, Fast Food, Gauntlet (Answer Software version) and Secret Quest.
Joust, Mario Bros, and Dig Dug are my favorites. You should also consider getting Pacman 4k. I think they sell cartridges somewhere. The ROM is also available. It is much more similar to the arcade game than what they had there before.