I only ever did serious grinding in the first three generations.
In gen 1 I either went through the League or did it in the postgame dungeon. The latter in particular had easy access to both Chansey and Rhydon who both gave pretty good Exp. The Exp-Share was only minorly useful, though.
Gen 2 happened to be the generation where I managed to get the most Pokemon up to level 100 (multiple boxes) despite the level curve being so low. I mainly went through the League and did some training in front of Mt Silver. I think the Exp-Share was a lot better here, though. So leveling was made a lot more accessible.
Gen 3 (Sapphire in particular) I spend mostly grinding in the League and Sky Pillar. but I also spent some time preparing for the first League playthrough right at the exit of Victory Road.
I can't really say which one was my favorite, though. Going by the numbers you'd think gen 2...
I am used to gen 4, so gen 4. In general, I didn't really need grinding and I often prefer to loose in a battle, change strategy or using different instruments more than just leveling up. I often grind too and when I do that, I generally rely on grass encounters, trainers skipped and devices like the Vs. Seeker. So, I try to do that in other pokemon games as well, but I feel comfortable in gen 4 because I remember places and some trainers too.
d Ambipom @ Level 100: 2,188
Gen 6 and 7. You can level up multiple Pokémon simultaneously with the modern Exp. Share, but you can also toggle it off and train individually if one of your Pokémon is getting behind, due to growth rates or whatever reason, and you don't want the rest of your team to overlevel (without having to box them).
Also the Exp. scaling system, introduced in Gen 5??? helps low level Pokémon catch up quicker.
I fell for the old "Beat the Elite Four x times with Lv. 100 Pokémon and y happens" ruse on Emerald so a lot of lv. 100s on there. Quite a few solo runs with Swampert too.
Gen IV was sort of my peak in terms of knowledge out of the three I played up until recently (Gen III-V). I like HGSS for EV-training as there are so many routes there's at least two good spots for each EV. Platinum for grinding though, Vs. Seeker is so much easier than PokéGear (although taking the time to clock-cheat and get all 16 gym leaders in one room, take a picture, then obliterate them all is fun!). Actually allowing clock manipulation makes HGSS more fun though, quite easy to grind up loads of Apriballs once you get to Azalea Town.
In Gen III I really like the Kindle Road for grinding before the Pokémon League. The steps from between the two Fighting-type specialists to the hot spring and back are enough to recharge the Vs. Seeker which is ideal. Especially when you want to have yourself a Dragonite!
My favorite old generation to train Pokémon was generation 7, or Pokémon Ultra Sun and Ultra Moon, mainly because of the Exp. Share that made leveling Pokémon to level 100 much easier and the fact that I didn’t really have to do much training at all with Pokémon stored in my boxes. Instead, I simply dropped them off at Poké Pelago for a few days to level them up while I played the game. I loved the Alola games so much!