Hear me out on this one: the original Pokémon Emerald. And yes, I do say "original" because Delta Emerald never became a thing on the 3DS. I want to mention that something being overrated doesn't mean it's actually bad. If anything, the word "overrated" is ironically overrated itself. All of that being said, I can't think of another Pokémon game, or many other games in general, whose public appeal is so highly acclaimed relative to the genuine quality of the actual product. At first glance, this might not be very apparent as to why. Out of all of the "third version" core series Pokémon games, Emerald is probably up there with Platinum as one of the games that improves upon its first two versions the most. It also added the Battle Frontier and a number of other things.
But that's just it- what exactly are those other things? At the time of its release, Emerald was notable for finally granting access to all 386 Pokémon in the franchise. While it's true that some of them were made easier to obtain, they weren't impossible to obtain before. In fact, limited-time events not withstanding, every single Pokémon could already be obtained in at least one of either Ruby, Sapphire, FireRed, LeafGreen, or Pokémon Colloseum. The sole exception I can think of off the top of my head is Lugia, who for all intents and purposes could be considered the Gale of Darkness version exclusive to Ho-Oh's Colloseum appearance once both games had been released, but even that's a non-factor in the off chance you have or had access to the Navel Rock event item. That brings me into the added problem of Emerald still not fixing many of these other Pokémon's accessibility issues in general, as perhaps more than any other generation you're required to do a ton of trades between games to complete the National PokéDex.
Okay, so if no new Pokémon (in the context of Gen 3's full lineup, anyway) were added, what new stuff aside from the Battle Frontier was added? There was the Faraway Island Mew event, but that's another event. There was some new Move Tutors added, but all of them are either the same Move Tutors as FireRed & LeafGreen (minus Frenzy Plant, Blast Burn, and Hydro Cannon) or are locked into the Battle Frontier. There was the addition of Juan and Wallace being made the Champion instead of Steven, but this is functionally more of a side-grade than an actual upgrade since Steven was already in Ruby & Sapphire and Juan's only unique Pokémon on his iteration of the Gym Battle team compared to Wallace are used by other Trainers too. It was the first game to introduce Gym Leader rematches, I suppose, but those aren't unique to Emerald anymore. But hey, at least you can go back and collect the Master Ball this time if you forgot to the first time like in Ruby & Sapphire! ...yeah, that should have been acknowledged in the first two versions anyway.
That just leaves us with the Battle Frontier itself, which honestly isn't even that interesting to me. Preparing for the full extent of what the Frontier provides is agonizingly slow, and all of the facilities are just different takes on something you've already spent the rest of the game doing. I don't want to reward myself for doing a bunch of repetitive battles by... unlocking more repetitive battles. In the games specifically- sorry, single game, because none of the Frontier Brains ever show up in future games- none of them have any sort of established backstory or reason to remember they exist. All in all, I just don't see any appeal from this game if you're looking for something actually new, which is why I think it's the most overrated Pokémon game.