Two locations that should definitely be added are Mt Silver Donphan/Dodrio Hordes as well as Golduck/Seaking Hordes when surfing inside Mt Silver Cave.
Donphan/Dodrio Horde needs a mold breaker mon, as donphan has sturdy. The best choice for that would be modest, 31spatk, specs surf Rampardos. It only needs 600k EXP for Lvl 100, so its pretty easy and quick to train. It very rarely happens that a Donphan survives, maybe like once per hour. Once Basculin gets mold breaker, it will definitely be the optimal choice for this spot, but the difference between wiping every horde out or just 799/800 is almost non existent.
To unlock the full potential of horde I definitely suggest using the EXP reamplifier if you can afford it, as it shares 75% of the EXP among party member, compared to only 50% from EXP shares.
Arguably the best EXP spot if you train a mon that cannot wipe hordes out on its own. With EXP reamplifier + dono, donphan hordes drop slightly above 8k and dodrio hordes drop slightly above 7k.
Golduck/Seaking Horde is more niche as your mon needs to be able to wipe those hordes out, with golduck/seaking being lvl 60+. Most of the time, the mon you are wanting to train needs to be lvl 80+ with an powerful aoe move like hyper voice, earthquake or discharge. Whenever you train a mon that meets those criteria, you should definitely start training them with the donphan/dodrio horde, then swap to golduck/seaking horde once its able to defeat those, as they drop like 10-11k EXP with dono+exp reamplifier.
Definitely the best spot for EXP for any mon that can wipe this horde out. Even if you need to be around lvl 80 to do this, dont forget that level 80 is only the half way point on your way to level 100, as 50% of total EXP are needed for the last 20 levels for most mons.
Those are the spots that I used for EXP when I still ran my leveling service, with the donphan/dodrio spot yielding me around 1-1,2M EXP per hour.
It goes without saying this is a super endgame approach to leveling, but its just the best way. I dont have the numbers without dono+reamplifier, but I tested out the other spots aswell.
You just always default to the Donphan/Dodrio Horde, which, compared to knocking out Rapidash or Tentacruel hordes with the to be trained mon itself, drop around the same amount of EXP, but without the prerequisite of the mon that needs to be trained to know a certain move or be a certain level. And if it does, chances are you can just switch over to inside mt silver cave later down the leveling grind anyway for a much higher EXP gain. You'd need repels to reach the water inside mt silver cave without encounters efficiently, but there arent much steps needed to go there, so the repels are definitely worth their money here.
Its a generalist approach with essentially no downsides and a lot of upsides. The only downside would be to get yourself a Rampardos and getting the EXP reamplifier to unlock the Donphan/Dodrio hordes full potential. If you ever plan to do a decent amount of leveling, you should consider investing into a modest, 31spatk Rampardos and choice specs for it, which you should have anyway.