Pong
Pong is a game commonly used to teach new Scratchers how to use Scratch. AngieJones made a pong game called Pong Easy Bro in 2017, which became frequently referenced amongst the remixers. It has its own remix tree, along with some standalone and remix tree derivatives.
Pong projects
Pong remix tree
Related projects
Pong in the remix tree
(There's also r/snapple:) and its remices, but those become increasingly less game-like, and so it's diminishing returns to include them here.)