I only played Gen III-V so I might double-up in Gen III/IV to talk about the remakes.
Gen 1. I think this should be classed as a feature, But I liked that the starters where basically difficulty levels I.E Grass - Easy, Water - Medium, Fire - Hard
I've never thought about this, not sure how much I agree with it though - yeah that's how they work out in the beginning but once you beat Brock, you can get Pikachu for Misty and after her Charmander's family dominates.
Gen Five - I'm also gonna choose seasons for this generation, it might some areas look really cool and Game Freak implemented it well with Deerling.
I definitely agree on this, although its also the thing that makes me want to replay them slightly less - now I have to have a job and stuff it takes me around a month to complete a game, so I have to pick which month I want to start in to decide how it impacts my gameplay. It does also mean it'll take longer to get super-repetitive though I guess.
Gen Seven - Poke Ride, something I've wanted for years and was actually really practical too, as it replaced HMs.
This has me wishing I could afford a 3DS, even if all the new pokémon of 2 generations seems a bit much. My Gen V runner-up would have been reduced HMs.
So, my list:
Gen III:
FrLg - reduced HMs! It seemed so leisurely and more open than other games, especially as the only gym badge you need to get preventing you from making Blaine your 3rd gym is Koga's. Last save my order was Brock-Misty-Koga-Blaine-Sabrina-Erika-Lt. Surge-Giovanni, nice to be able to mix things up.
Emerald - Battle Frontier! Great way to have enough to do in post-game without having to revisit Kanto yet again or have a lacklustre area like Sinnoh's Battle Zone.
Gen IV:
DPPt - Physical / Special split! I wasn't aware of how types / contact worked in Gen III, finally making it follow logic (mostly) just makes everything so much clearer for me. Close runner-up would be Day / Night, I love the night-time themes.
HgSs - Walking pokémon! I mean seriously, what else? Expanding on those images upon entering certain locales from FrLg was fun but c'mon, walking pokémon!
Gen V: Seasons, as I outlined earlier. Simultaneously my favourite new feature and the thing that prevents me from going back to them.
As an aside, I'm not that big of a fan of Cynthia's appearance. I guess there's no other stronger trainer from in-game they could have added, but I really liked that Black and White seemed so self-contained. The introduction of the other regions' pokémon in B2W2 kinda made them less special to me as well.