I think gen 4 mechanics wise was really good with the physical/special split, and that gen 5 had like an actual storyline that was a little bit beyond "big bad team tries to take over world" and at least attempted to raise moral questions and the like.
but man gen2/3 are hard to beat, story and post game content are really really good and the new mons were great. i actually really hate gen 1, can't place it but there's something about it that rubs me the wrong way besides my blue cartridge being a glitchy piece of shit.
never played past b/w
edit: also holy shit emerald post game is honestly so fun but fucking infuriating