4 was the one I had the most enjoyment of all, I actually enjoyed D/P when I played it, back then I didn't care about how slow it was, but Platinum went and not only fixed everything, but made it even better (Cyrus also isn't an absolute retard who seems to not even know the Lake Trio can stop one of the Creation Trio when they're alone, in Platinum, plus the fact all seems lost until Giratina arrives is fantastic in my opinion). HG/SS were the pinnacle of Pokémon for me, as well, so I can't put 4 anywhere but the top. The spin-offs were also some of the best the franchise ever had, with PMD2, Ranger 2 and 3, PBR, and PokéPark Wii.
3 and 2 are close together, and they're up there because I think they're both great in how they play, plus the music is wonderful in both, and I guess some of it can be attributed to nostalgia as well, but 3 includes a bunch of my favourite Pokémon, more than 2, and 2 is somewhat overshadowed by the fact HG/SS exists. Crystal is still one of the best, though. The spin-offs were also fantastic, with 2 having Puzzle Challenge and Stadium 2, and 3 having the Orre games, PMD, Ranger, Pinball R/S, and Trozei.
I actually had a bit of a hateboner for 5, up until I replayed it a couple of years ago, not for any reason like the lack of older gen Pokémon, or the story, but mainly because I think I burnt myself out while grinding in B/W on like... My third playthrough close after its release. I have no idea why, especially since I was incredibly hyped before its release, it was really stupid. I've only replayed B/W so far, and while I liked it, it hasn't overtaken the top 3 yet (which is odd since it gave me my favourite PokéGirl of all when I first played it, too, and B2/W2 made her even better). However, grinding in gen 5 (or at least in B/W) is really painful, so there's that. The spin-offs are another reason why I put 5 lower than the top 3, they just weren't that great, especially PMD Gates and PokéPark Wii 2, but Rumble Blast was pretty good.
I actually liked the gen 7 games (let's not talk about Let's Go, even if it's technically gen 7, sadly), that's why I have it above the last 3. I really liked a lot of the designs, I liked the Totem Pokémon idea, and one thing that really stands out is the fact that the game has some difficulty for once. After the disaster that was gen 6, the games really needed difficulty, and when I have Hala in US/UM almost beating my ass because my team is half Dark-Type, or Guzma in Po Town actually beating me down with his Masquerain and Pinsir, or a few of the Totems beating me in both games, it was a welcome change for me. Sadly 7 doesn't really have spin-offs at all, except the abortions that were Rumble Rush and Quest.
The last three should be obvious, I only have 6 above 8 because, while both were disastrous generations, ORAS exists, and even though it was a poor remake after the bar they set with HG/SS, it's still better than the fiasco that is Sw/Sh. X/Y is better as well, in a way, but the fact Game Freak left so much to be desired from that gen through skipping to gen 7 a couple of years later leaves a very sour taste in my mouth. As for spin-offs while 6 had Battle Trozei, Super PMD and Pokkén for good ones, Shuffle and Rumble World were absolutely terrible.