I was requested to comment here.
I'm happy to help, but I'm not exactly sure what my input is needed on.
As mentioned, Pokemon, especially in the anime, do often seem to mutually understand one another, even when they're different species & may have had little to no experience encountering Pokemon of the species they're communicating with.
Ergo, Pokemon not understanding human languages is the exception, since they either all share 1 language -Dubious, given how enforced them saying their own names is- or they have different languages, yet many often understand Pokemon without difficulty.
Team Rocket's Meowth did learn human languages with prolonged effort. IDK if this means anything else for his species; What if other Meowth could learn human languages just as he did? Does that improve how we rate their Intelligence?
& as mentioned, he's exceptionally intelligent for consistently providing many of the Team Rocket Trio's plans, many of which involve building fantastic technology.
& of course, there's that Pokemon readily understand commands from humans, even if they've never been told the name of that move before, never heard the name of it... but this could just be plot necessity, or maybe some kinda "magical mutual understanding" facilitated by a bond between Pokemon & humans/trainers, as such bonds are a prevalent theme in Pokemon.
Still, I think there's also been instances where wild Pokemon that are practically strangers understand what humans ask of them with little to no context, so they somehow do understand language strings they should have no linguistic frame of reference for understanding.
To put it another way, if you ask a Mantyke to push a rock, & it does, why does the Mantyke know human words like "push" & "rock", especially if it's never met or heard a human before?
In terms of Intelligence feats.... Well, there's Clefairy which could build a spaceship, but that could be attributed to Gen 1 Anime weirdness, were it not for it coming up in the Sinnoh Anime, too, I think?
There's probably several intelligence feats in the Pokedex. Lapras, Dragonite, & Mew are just a few examples off of the top of my head of Pokemon noted for being intelligent.
Are we looking for?:
A. Feats of Intelligence, be they on-screen or in statements? ("X did Y" & such.)
B. Statements about how Intelligent a Pokemon is? ("X is smart"/"X is unintelligent".)
What is our goal & what is more important to it right now?
That said, beyond Pokedex knowledge, scattered bits of anime knowledge (I've watched a lot of the anime, but I don't remember it well, much less many feats.), my knowledge is a little lacking. There may also be text dumps of some of the games online that could be useful.
It might be easiest to determine some species to focus on, since Bulbapedia listing & describing Pokemon by their notable appearances on their pages, as well as providing detailed episode summaries & links to galleries makes seeking out information by species a little easier.