there's an intermission/'ending of sorts after stage 20 in Dr. Mario, then it repeats.
Game and Watch Gallery 3 probably has a bit of variance depending on how you want to define "completion"
Unlock Hard mode in each modern game by getting 5 stars
or
Unlock all the bonus games (50 stars for all the bonus games in the main menu, or 60 stars for the bonus games + reverse Fire)
or
Get the credits (100 stars)
or
Unlock all the museum entries (120 stars)
The only certain death in the universe is Mother Hitton's Littul Kittons.
Not changing this line until I beat Seven Heroes (2/24/07) - Fulfilled 2/20/18