Pokemon (2019): Harukanaru Aoi Sora

The episode features the main character, Pikachu and his mother. That's it. There are no other recurring characters and no other Pokemon owned by Ash, so a person could watch this special without any prior knowledge of the series. It also won't spoil anything, it's entirely standalone. Ultimately it's just another adventure with Ash and Pikachu in the fantastical world of Pokemon, but it's very well executed. The story of the episode is pretty interesting and the art style is on point. It's more similar to the movie continuity, for those who have seen the movies. Ash and Pikachu were their usual adventurous selves, thoughyou do see a side of Ash that doesn't show up as much anymore. I think it's worth a watch regardless of what your opinions are on the anime as a whole. It's pretty short but still pretty good.