Here's the definitive opinion on each generation on its campaign, features/mechanics, and competitive meta with a brief takeaway after each.

Gen 1: Good campaign, shit features/mechanics, shit meta. Takeaway: Deserves hella credit for starting it all, but no one should play these out of anything but nostalgia now.
Gen 2: Great campaign, good features/mechanics, good competitive meta. Takeaway: Was the perfect follow-up to what came before and laid a lot of pavement for what was to come, despite some pacing issues.
Gen 3: Good campaign, good features/mechanics, good meta. Takeaway: Black sheep of the family in being the first and only reset button in terms of generational transferability but deserves credit for experimentation of mechanics that have become staples (e.g., Natures, Abilities, in-game "competitive" facilities).
Gen 4: Good campaign, great features/mechanics, great meta. Takeaway: Most balanced generation. Campaigns were content-rich, features mechanics took a big leap forward in physical/special split and WiFi, and meta felt balanced with a wide array of viable strategies and Pokemon.
Gen 5: Great campaign, great features/mechanics, shit meta. Takeaway: Best campaigns of the series with best villains and in-game vibe/content. Great feature/mechanic additions with reusable TMs and Hidden Abilities. Shit meta due to weather wars and controversial team previews.

[WILL CONTINUE IN COMMENTS]