I remember trying a demo of Pokémon Rumble on 3DS but not being able to think much of it because of how limited the demo was. That aside, I think my favorite spinoff has to be Pokémon Snap since its the only one I've played for a considerable amount of time on a real N64.

Pokemon Mystery Dungeon and Pokemon Ranger I mentioned since I also played those a bit thanks to my other brother but not that many times (Pokemon Mystery Dungeon on DS/GBA I think didn't have extra save files so I couldn't really play it and do progerss as I didn't want to mess with my brothers save files, and Ranger is also like dash where you have to go crazy with the touch screen), but PMD is definitely the most well received alongside Snap, which is funny considering Mystery Dungeon is basically a game series of its own (the first one that was based off Dragon Quest and originally Japan-only is getting a remake soon), so Snap is the other succesful spinoff that had an idea of its own...and a sequel on Switch no one remembers.

I think the only other spinoff I could say I played extensively is Hey You Pikachu only by chance of the N64 I had for years happening to be one that included it with the mic and everything - and sure was a neat thing to try as a kid, but in retrospective can see why it wasn't that popular either when it was basically about interacting with Pikachu through the mic and nothing else. Funnily enough that game's developer, Ambrella, also ended up making Pokémon Dash and Rumble amongst some other Pokémon spinoffs.

Thanks for reading and for your comment! I'm pretty sure there has to be better stuff on the Pokémon spinoff bin than this (besides PMD and Snap which both I have to give a due revisit someday) - Pokémon Pinball intrigues me for some reason and the GC games are technically spinoffs but are alike the main games where you get Pokémon and fight with them (only that the way to get them and the structure is different).

...Oh, right, Pokémon Trozei on the DS also exists. Been playing that one during power outages, although not sure if I'll talk about that or another DS game for next time since Trozei is similar in how there isn't THAT much to describe beyond the core gameplay loop