While I cannot list all of them, I guarantee I can find a use for every pokemon when they were introduced.
Unown.
I have a rough time believing this, unless "a use" includes memes, folklorical reference, or straight experimentation.
Well they didn't say it was a "good" use.
And I'm only being semi-sarcastic. Like, sure, as long as the Pokemon can learn multiple moves you can come up with a strategy it can use... but whether it'll do anything to make it viable is a separate question. Like, let's take Scatterbug. Now it's much like the other basic stage gen bug where it learns practically nothing and lucky if it gets any TMs, Tutors, or Egg Moves. Heck, Scatterbug was dexited so I'm looking at it's Gen VII movepool. Still, if asked to try and make it "useful", here what I would do:
Ability:
(If Double Battle & available: Friend Guard)
(If Single Battle or can only use standard Abilities: Compound Eyes)
Held Item:
(If has Friend Guard: Zoom Lens/Wide Lens)
(If has Compound Eyes: King's Rock/Quick Claw)
Moves:
(If Double Battle: Rage Powder, Stun Spore, String Shot, Bug Bite)
(If Single Battle: Poison Powder/Stun Spore, String Shot, Bug Bite, Tackle)
Strategy: Be as annoying as possible.
In a Double Battle use Rage Powder every turn to make the opponent waste at least a single turn taking out Scatterbug. If opponent's Pokemon are immune to Rage Powder (meaning they're likely a Grass-type or have Safety Goggles thus immune to Stun Spore too) try slowing them down with a String Shot. If you somehow are able to get their active Pokemon all to -6 Speed (or to a low enough Speed you feel comfortable at), I guess go on the offense with Bug Bite (though if you suspect their Pokemon is holding a Berry you can try a Bug Bite early to get rid of it). Having access to Friend Guard is nice as it reduces damage to likely your main attacker, though Compound Eyes would make Stun Spore more likely to hit.
In a Single Battle try to hit it either with Poison Powder (just accepting it's likely fate of fainting before doing any significant damage so try and leave a lasting impression) or Stun Spore (if you want to to actually try and hopefully this will buy Scatterbug some turns if it's not OHKOed). Eitherway, next decision is another multi-choice: String Shot (wanting to try and set up advantage for the next Pokemon out after Scatterbug) or Bug Bite/Tackle (actually try fighting back, though once again if you suspect your opponent has a Berry you can use Bug Bite to get rid of it). If you went the String Shot option and got the opponent's Pokemon down to a Speed you're comfortable with and still not fainted (you may want to check your opponent didn't fall asleep... or laughed themselves to death; remember if they did either that's a forfeit which means you won with Scatterbug!), than go nuts with Bug Bite/Tackle.
Now, is the above a good solid strategy? DISTORTION WORLD NO. Even in Little Cup Scatterbug ain't surviving for long enough to start actually attacking; lucky to even get a String Shot off. However, as basic and probably not very effective as this is, for Scatterbug, this is a "use" for it.
Of course, Scatterbug is """
lucky""" to have access to some useful Abilities and Rage Powder for use in Doubles. Other gen bugs not so much (though a few get Electroweb which Scatterbug doesn't for some reason, but at most that just replaces String Shot as the Speed decreasing move). And then we have Pokemon like Unown where the ONLY choice you have (aside not using it, a choice you should always make) is what item it holds (well, I guess also Hidden Power's Type though that's a one-time decision thing) and I don't know what non-joke item I could even give it (maybe just a Sitrus Berry? King's Rock/Quick Claw if you're feeling really generous?).
