It's a toss-up between several games: Pitfall!, River Raid and Super Breakout for the Atari 2600 are all up there, as well as an obscure Atari XL/XE game called Dan Strikes Back.
But I'd have to say my number one game from Way Back When was Lode Runner for the Atari XL/XE. I dimly remember Dad buying it and bringing it home. At that time, we were only just managing to keep food on the table and fuel in the car, so getting a new game was a big event. Solving the puzzles was a family affair - I remember we were all dreadfully stuck on one level quite early in the game, until Mum had a brainwave and we all kicked ourselves. (This was back when my parents still played video games - a long time ago...)
It was a great family game because it was so relaxed about difficulty levels. The little ones could slow the game right down, so that they had plenty of time to react, while Mum and Dad and I could play it at the top speed. If you were stuck on the level you were on, you could jump to the next level with Ctrl+U. It even had a keyboard shortcut to give yourself extra lives! No, not a cheat, it was listed in the manual along with all the other keyboard shortcuts... (I think modern game design could learn a lot from Lode Runner in this area, but I know that saying this will make a whole lot of hardcore gamers run out screaming, "NOOOOO they're dumbing our games down...!")
We all played the heck out of Lode Runner. Soon we all knew every one of the 150 included levels like the backs of our hands, but we kept playing it again and again. Partly because this was long after Atari's hey-day and we only had a handful of games for our obsolete computer (a new computer was out of our price range at the time), but mainly because of the level editor. The level editor was a breeze to use, and could do everything that any of the built-in levels could. I have no idea how many levels we created over the years, but there must have been dozens, maybe even hundreds. We didn't have a disk drive to save them, so it used to lead to terrible arguments: "You turned the Atari off and now my level is gone! I wasn't finished with it!" I believe Mum and Dad's house is still littered with meticulously drawn diagrams of particularly successful levels, although we never actually went back to any of the levels we'd saved so carefully.
