Well, I'll try come up with the best damage simply with move, ability and item. :D
Firstly, I'll find the Pokemon with the highest Attack/Sp. Attack stat - which is Mega Mewtwo Y, with a whopping 194 Sp. Attack! :o
Now, Mewtwo Y's strongest STAB move is Future Sigh with 120. Two incredible values!
The damage calculation formula is the following:
>Damage = ((((2 Level / 5 + 2) AttackStat AttackPower / DefenseStat) / 50) + 2) STAB Weakness/Resistance RandomNumber / 100
Now, inputting the values, let's say we are calculating a Lv100 Mega Mewtwo Y, with a perfect Sp. Attack stat (447!) using Future Sight on a Lv1 Weedle with a completely useless Sp. Defense stat (4, the poor thing D:) and will also receive a super-effective (x2) bonus. (There's not really any weak Pokemon that have a 4x weakness to Psychic, so don't blow your heads of pwease. :3)
Damage = ((((2 100 / 5 + 2) 447 120 / 4) / 50) + 2) 1.5 2 100 / 100
Damage = (((42 447 120 / 4) / 50) + 2) 1.5 2 100 / 100
Damage = ((563,220 / 50) + 2) 1.5 2 100 / 100
Damage = (11,264.4 + 2) 1.5 2 100 / 100
Damage = 11,266.4 1.5 2 100 / 100
Damage = 33,799.2
Damage = ~33,799
That's enough in itself to faint (or should I say murder) that poor little Weedle. Unfortunately though, this can't be increased any further though because Mega Mewtwo Y doesn't have any stat-boosting abilities, and is already holding a Mega Stone, unfortunately. But I don't think that matters since that's well in truly enough damage.
However, there's still this wonderful method that Shuckle can achieve inflicting, wait for it - 481,266,036 damage! This method is a heck of a lot more specific, but evidently, it works. Though I think a mere Tackle can overcome a mere Caterpie, it's always nice to have some fun though. :P
NOTE: As you can see, bigger amounts of damage can be achieved. My answer sucks. Boohoo ;-;
Hope I helped. :)
Sources: Bulbapedia. Lots and lots of Bulbapedia. And the above link. And Experience. And Frenzy Torterra. Yeah. I have a lot of sources. :P
