1. Gen 2- It's a close call between my top 3 but I have to go with Gen 2. This is the Gen that got me started in Pokemon so the new mons introduced in Johto were my Gen 1. My friends and I grew up playing these games as kids. Gen 2 introduced so many of my favorite mons, such as Tyranitar, Sneasel, and Houndoom. And you still can't beat getting to travel back to the Kanto region and getting to battle Red for the first time.
2. Gen 4- Again close call between 4 and 6 for this spot. I first played Gen 4 around the time X and Y came out. I hadn't played a Pokemon game since Gen 3 at that point and I'm very glad I decided not to skip directly to X and Y. I absolutely love the music in this game, the Jubilife City music is my favorite track from any game. The story in this game was great, the design and history of the region, as well as introducing some awesome new mons like Empoleon. This is the gen that gave my beloved Sneasel an evolution into my favorite mon, Weavile!
3. Gen 6- Having been a longtime player when Gen 6 was released, I was instantly blown away by the new 3D graphics! I love that the Kalos region is based on France! The music in this game was once again fantastic. I love so many of the mons introduced in this gen: Greninja, Goodra, Aegislash, Trevenant, Florges, Sylveon, etc.
4. Gen 7- I love all the features Sun and Moon brought to the table, from even more graphical improvements to a more story-driven playthrough. Didn't leave quite the same impact on me as my previous 3 though.
5. Gen 3- This is the generation I probably spent the most time with due to playing these games at an age that was prime for video game playing. I spent a lot of time battling and trading with friends during this gen and I thought the sevii islands post game in FRLG was a nice addition. Some of my favorite Hoenn mons are Blaziken, Ludicolo, Breloom, Flygon, and Shiftry. Before ORAS I kinda forgot about the Hoenn region itself though, and the music in this game was far from my favorite.
6. Gen 1- I ranked this so low mostly because my first experience with Gen 1 was through Fire Red version. I have since gone back and played through the original Red and Yellow versions, but had I started with Gen 1 in the beginning I may have ranked it higher.
7. Gen 5- Admittedly this is the gen I completely skipped over during my time away from Pokemon, and as I mentioned when I came back to it I went right from 4 to 6. I've since familiarized myself with the mons introduced in this gen. Not a big fan of any of the starters. The only mon I really like from this gen is Excadrill.