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Off the top of my head, the Deerling seasonal thing is pretty neat, and I used to have more fun with Castform than I had any right to. The thing where Eevee can evolve into eight different things is pretty neat but everyone already likes that so I won't count it here.
Since Deerling/Sawsbuck seasonal forms were already mentioned, I'll have to go with Rotom and it's forms (RotomDex is not included here since I hate that annoying thing). You basically get 6 Pokemon in one and you can change the forms depending on what type you need the most in different portions of the game. Most of the Rotom forms are also very usefull competitively in their respective tiers. My favorite forms are Heat, Mow and Frost.
Shedinja is many gimmicks in one. The way to even obtain it is pretty unique, and it's the only instance of getting two pokemon from one pokemon (except using the DNA Splicers). It has a unique ability and the unique property of having only one HP.
The way branching evolution is handled, generally speaking. I think it makes good use of the game's general ecosystem without being overly annoying, in my opinion. It's just neat and fascinating, how exposing a Pokémon to different phenomenons will get them to adapt to different things.
Coming in second: using Pokémon as mounts! Just hoping ShSw is gonna keep it in Let's Go manner, instead of restricting to a specific Pokémon per function like SM/USUM did.
I absolutely loved Inkay's evolution, and well, forms in general. Lazy, in a way, yes. In another way, like branching evolution, I find it fascinating that Pokémon vary this much, from region to region.