So, you want to be the best? Like no one ever was? You've got a long road ahead of you. The post-game content for Let's Go, Pikachu and Eevee is vastly expanded from it's humble roots in the Red, Blue and Yellow versions. One thing does remain the same, your first port of call after taking down the Elite Four and your rival.
After you have beaten the Elite Four, Cerulean Cave is opened up to you. This should be where you go straight away, after a small pit-stop at the Cerulean City Pokémart. This unassuming cave is where most of your post game time should be spent. Cerulean Cave not only houses Mewtwo (as well as a secret challenger) but is also the best place to level up your monster friends. Here's how to do it.
The first thing you will want to do is stock up on Ultra Balls at the Cerulean City Pokémart, get as many as you can afford at the time, which after your undoubtedly successful run through the Elite Four is a decent amount. You can also buy some Max Lures to raise the spawn chance of the specific pokémon we are after.
You will want to Sea Skim over to the right hand side and go up that ladder that's there. Here is where the fun begins. Catch chain all the Chansey you see. Spraying a Max Lure will help raise the spawn chance but so will successful catches ranking up your chain. Chansey has about a 10% spawn rate without the Max Lure.
With your Max Lure activated, catch every Chansey you see and only Chansey to rack up a chain. Aim for nice, good and great throws to add more multipliers to the catch.
If you are in an extra hurry or just want the experience to come faster, grab the second joy-con and get a synchronised bonus added to the chain. Before too long you will be racking up at least 30,000 experience per catch.
If the Chansey breaks free the first time you throw a ball, just run. Your chain will remain intact and that is what is going to get you more and more experience. Spend a couple of hours in Cerulean Cave catching these egg-bearing wonders and hit max level with at least a couple of pokémon before you can say "Pikachu is a pants mascot."
You will also have so high a chain that your spawn rate for shiny pokémon should also be making them appear more frequently. Also the higher the chain the more likely you are to get a perfect IV pokémon.
Fill out your pokédex
Before, during or after your Chansey chaining you might want to fill out your pokédex as well. This involves catching at least one of every species of pokémon in the Kanto region. A lot of previously uncatchable pokémon will spawn in Victory Road as rare spawns, including Hitmonlee and Hitmonchan. So don't worry too much about which one you picked from the Fighting Dojo.
The map above has a rough guide to where most pokémon will spawn.
Some specific pokémon are version exclusive and they are as follows:
Let's Go, Pikachu Exclusive Pokémon:
Oddish, Gloom, Vileplume, Sandshrew, Sandslash, Growlithe, Arcanine, Grimer, Muk, Scyther, Manky and Primeape.
Let's Go, Eevee Exclusives Pokémon:
Bellsprout, Weepinbell, Victreebel, Vulpix, Ninetales, Meowth, Persian, Koffing, Weezing, Ekans, Arbok and Pinser.
If you are also still playing Pokémon GO, you have access to all of these pokémon and can transfer them to your GO Park in Fuchsia City to circumvent the exclusivity. You can also trade with friends who have different versions.
Filling your Dex will get you the Shiny Charm which raises the spawn chance of shiny pokémon.
Defeat the master trainers
Master Trainers are a completely new addition to the Kanto region, spending their whole career training one specific type of pokémon.
There is one for each of the 153 pokémon in the game. Including Mew, Meltan and Melmetal.
These Master Trainers can be found all over the region. Not all of them are going to be battles, some of them you just have to show them the pokémon they claim to be the master of. But the majority of them will be fights. One on one battles with no items and more often than not the pokémon you are battling will have moves you will not be prepared for.
If you want to stand a chance in these battles, go in expecting the unexpected and with a pokémon that you have beefed up with candies and preferably a mega stone and a couple of TMs for unique moves, or if you have it, try the Alolan Form for extra style points.
After you have bested six of these trainers you can also battle Red, the protagonist from the original generation one games. He is no push over, as you may expect.