One of my favorite regions is Sinnoh. for sentimental values. Surprised that same as you, Pokemon Yellow was the first Pokemon game I had played(I played it in 2006, so I was about 6-7 years old at the time) and I did played Pokemon Emerald back then as well(I'm still doing so, as part of a playthrough series on my YouTube channel. Just need to make time for it again.), but I watched one of my older brothers who came by for a visit one time (since two of them live used to live with my dad before moving out to have their own lives, while the third and last older brother, who is older than me by 13 months, lives with me, my mom and my other siblings) played Pokemon Pearl, and after asking, I managed to play it, and I enjoyed traveling through the region, seeing all the new Pokemon(since I had never played played Generation 2 before.) Plus, the Diamond and Pearl era for the anime helped made my love for Sinnoh grew bigger, especially since I enjoyed Paul's character and the rivalry he had with Ash.
The Johto and Alola games are also my favorite games because of the stories they told, the features they had, and the characters, even though the Rotom Dex could be annoying at times, I still like it.
However, the region I thought looked amazing was Kalos. Sure, back then, I thought it was bland, just like how everyone else thought. However, looking back on it(especially since I'm writing the based on the Kalos games), I realized that underneath all the complaints is a region that is not only beautiful, but also has a lot of stories to tell and characters to help tell them. In fact, it's tied with Unova as my favorite region. It's even the reason why I decided to write the book based on Kalos first, instead of starting with Kanto, then making my way to Kalos.