My criteria was "design concept that comes through during how it's used in battle" to keep it consistent. There are others, but these are what I'd argue are the most apparent of each Gen

Gen 5 and 6 were not working very hard in this department. Gen 7 has a tonne that could be put here instead. For the other gens it's usually more about unique evolutions (Eevee, Remoraid) or form change (Cherrim, Castform, Pumpkaboo.) I feel Normal, Ghost and Fairy are the "experimental" types, even if there's only one Fairy here.

>Ditto
Blob with no moves of it's own besides the ability to morph into every Pokemon
>Smeargle
Painter that "sketches" moves to be able to learn every single one
>Shedinja
Shell that is immune to everything but dies instantly to things it's weak to
>Rotom
Ghost that inhabits machines and uses them to learn moves
>Basculin
Separate species of the same pokemon that gives you different abilities
>Meowstic
Different moves and abilities for the same pokemon based on gender
>Mimikyu
Pokemon disguised as another pokemon that uses that disguise to protect itself from damage

Discuss