Super Mario Brothers 2 on the old Nintendo
I actually found the original Legend of Zelda for the NES pretty easy. I also found Top Gun and Double Dragon really easy. Mortal Kombat was also fairly easy. After a couple days I could do all the fatalities in my sleep and beat the game with all the characters.
Do you remember getting locked in a cage by bishop Mandible. And his henchman Cob guards you while he goes to summon a demon.
Cob: They say it is death to look under a weaver’s robe. I’ve always been curious.
Bobbin: I’ll let you look if you pass me the staff
[cut to outside]
Cleric: Can’t you keep it quiet in there? It’s hard to concentrate with all that screaming.
In expert mode you get to see Cob sucked into the blackness under Bobbin’s hood. I think the non-expert mode is much funnier. And it’s a bit of a rip-off that you get a 1 sec extra animation that adds nothing to the plot.
I’ve already said PONG is #1 on my easy list, but how about DonkeyKong for #2 followed by PacMan #3
SuperMarioLand for Gameboy seemed pretty easy to me (and I’m terrible at these type of games, I’m pretty good at sports and racing games but supermario types…forgetaboutit). It wouldn’t have been nearly so easy if they didn’t give you a chance to win 3 extra lives every single level. I think by the time I actually finished it, I had something like 40 lives.
Those NHL games are all pretty easy. One of them, I think it’s NHL 98, all you have to do is skate down the boards and shoot it on the short side and you’ll score everytime. The only way these games are fun is if you play against another human.
Make that 5 extra lives per level. In addition to the bonus game at the end, there’s also at least one extra-life mushroom and at least one life worth of coins per level. There are actually even levels where you can collect about 500 coins, die, then re-start just before the 500 coins (lather, rinse, repeat). And then get your fireflower back before the end of the level.
Altered Beast for Genesis.
I remember buying my Genesis, getting home, excited because I had a game with it, sitting down…and playing right through it…I couldn’t believe I saved up my money for a system/game and in a couple of hours was done with the game. Luckily I had also purchased Lakers vs. Celtics, which kept me busy(along with John Madden Football).
Asteroids for the Atari 800 computer.
The Atari 800 had four joystick ports so they decided the should give you the option to play four player games. The key point is that it all four players played at the same time. All you had to do is get three friends and park back to back in the middle of the screen and play till someone fell asleep. Even one person falling asleep wasnt much of a challenge, three could keep playing almost as easily. On top of that you got an extra ship every 100,000 points or something like that, so not only could you play forever you could rack up extra lives forever. Once after we played for a few hours we just let go of the sticks and I think it took something like fifteen minutes just for the game to kill all the extra ships we had racked up.
Honorable mention:
4th and Inches
C64 football game, the computer could/would not defend the post pattern on a pass play. You could score almost any time if you called this play. The only reason I didnt call this the easiest game is that you did not get the option to call the post pattern on every play and you needed about 30 yards of field for it to work. If you could keep yourself from using the post pattern it was otherwise a pretty good game.
All of these examples are good ones, but the absolute unquestioned winner in the category of “easiest game ever” HAS to be Progress Quest.
I defy anyone to find an easier game. 
Mortal Kombat for GameBoy was pretty easy.
Baldur’s Gate: Dark Alliance for the PS2 was stupidly easy after a while. My games went like this:
First game: Finished in 12 hours
Second game: Finished in 8 hours
Third game: Finished in 5 hours.
Rinse and repeat for all of the difficulty levels. And I’m far from being a skilled gamer.
Ah, good point. I actually got quite far in that. I had it running straight for about 6 months.
What about super mario world for the Snes. How hard can it get with infinite lives? All you had to do is go through the ghost forest board about 10 times and get 99 lives.