It's interesting to see the progression in these games:
Robot Wants Kitty is practically a proof of concept with its bare-bones artwork and a relatively small game world, but establishes the basic "metroidvania" game progression.
http://jayisgames.com/games/robot-wants-kitty/
Robot Wants Puppy makes the art a little better, adds bosses, more interesting enemies, and a really cool kitten-based combat system. The only weak part is the over-reliance on keys, i think -- the whole point of this genre is using abilities rather than locked doors to gate the player.
http://jayisgames.com/games/robot-wants-puppy/
Robot Wants Fishy really brings the polish; the world design is more interesting, the art is better, tbe bosses are better. Introduce health points (in this case a shield good for one hit) and upgradeable items.
http://jayisgames.com/games/robot-wants-puppy/
The last in the series,
Robot Wants Ice Cream is on the verge of getting away from the roots a bit, but is still solid if you like the previous games. There's a crazy amount of pickups in this game since everything gets upgraded multiple times, and they probably should have been color coded, but it's still good. Now you have three hit points and can aquire energy tanks to power your special abilities. Progression requires more thought and exploration than before, but not in a bad way.
http://jayisgames.com/games/robot-wants-ice-cream/