First thought that came to my mind is probably games that weren't well known and by default doesn't have a lot of competition? A game that comes to mind is Roc'n Rope(1983). The Twin Galaxies record is 487k, a great score, but it's also the only score. With my limited experience with the game it tells me that, like Donkey Kong, it's the same boards that repeat in a cycle and the difficulty top out at some point.
More popular games that come to mind, that in my opinion, are "easy" to beat are Galaga (Fast Fire) and Gradius/Nemesis. In Galaga's (1981) case it comes down to knowing the patterns and knowing how to play the game. You have to memorize the bonus stages but after that it becomes a marathon. Fast Fire helps to give you the advantage and lowers the difficulty compared to the original.
Gradius (1985) is purely memorization. It's 5 stages, if memory serves me, and the game loops indefinitely, so once you've got the patterns/strategies down, it also becomes a marathon.
