Pokémon Ultra Moon
I wouldn't consider Ultra Moon my favorite game in the Pokémon series, as I enjoyed a select few titles even more, but it was definitely the most exciting structurally. After the series seemed to get itself stuck in a rut, gyms were replaced with island challenges and everything became a bit more interesting. I felt like the focus wasn't just on making my team stronger, as the island challenges often revolved around strategy and deeper-thinking as well.
I don't think that I would recommend Ultra Moon as an introductory Pokémon game, only because it plays so well in contrast with the history of the series, and new players wouldn't be aware of the advancements or references. However, for returning players such as myself, it is a great experience and a title that I am glad that I got around to.
The game can be purchased for 3DS here (as of 8/18/20): https://www.nintendo.com/games/detail/pokemon-ultra-moon-3ds/
Photos sourced courtesy of https://www.serebii.net/ultrasunultramoon/pics.shtml
Photos sourced courtesy of https://www.serebii.net/ultrasunultramoon/pics.shtml