Counting only main-series games (from most-liked to least):
1. Omega Ruby & Alpha Sapphire
2. Black 2 & White 2
3. Let's Go Pikachu & Eevee
4. Emerald
5. X & Y
6. Platinum
7. Black & White
8. HeartGold & SoulSilver
9. Crystal
10. FireRed & LeafGreen
11. Ruby & Sapphire
12. Gold & Silver
13. Diamond & Pearl
14. Sun & Moon
15. Red & Blue
16. Yellow
17. Ultra Sun & Moon
Note that the "dislikes" start after Gold & Silver.
Ultra Sun & Moon ranked lowest because they're the only Pokemon games so far that have outright DISAPPOINTED me. They didn't fix ANY of Sun/Moon's issues, and the additions don't make up for that. Likewise, while Yellow's an improved Red/Blue in some aspects, the anime theming detracts from it more often than it helps IMO. (Jessie and James really add nothing, for instance.) DP... Honestly, mostly dislike it due to Mt. Coronet. (Both mechanically just due to its size, and its role on the world map. Not a huge fan of regions like Sinnoh and Kanto that loop around a lot.) Aside from that, it's mostly this low simply because Platinum is a straight improvement in every way (similar to why Ruby and Sapphire are as low as they are).
On the other end... ORAS just felt to me the most "polished" Pokemon games so far. No real glaring issues, it's great mechanically, it improves on both RSE and X/Y quite a bit. Overall great games. BW2 probably had the best plot of any Pokemon game so far, and the changes to the world after BW were a very welcome addition. (REALLY wish Ultra Sun & Moon took the same approach.) Let's Go has me very excited for Sword & Shield (though I wish its artstyle was a little closer to Let's Go's), and they were very well-made games if you ignore competitive battling (which they did REALLY screw up with, so it's hard to forgive, but still), and the candy aspect.
As for spin-offs... I generally like them for the most part, though I haven't played them all (especially home console spin-offs like the Pokepark games). Rumble's probably my favourite spin-off series, though Mystery Dungeon (with exceptions for a couple of entries) and Ranger are very good as well.