I just received in the mail one of my favourite games of all time.
Originally Berzerk (I didn't realize the spelling until now) was an arcade game, and as well as the Atari VCS/2600 version here, it was ported to many, many systems, both officially licensed and ripped-off.
My first in-depth experience was playing a rip-off. Amok on the Vic 20.
Man, I loved that game,
It's a simple overhead view maze shooter, but with directional shooting and you can get the robot bad-guys to self destruct by walking into walls.
Considering the Atari 2600 and the unexpanded Vic 20 both had minimal RAM (the VCS only has 128 bytes of RAM on board!), there's surprisingly a lot to the game, with 1,024 maze layouts (876 of which are unique) according to Wikipedia.
Between this and Galaga, I am not sure which comes top, but I can play and play quite happily, decades later!