I found this on Bulbapedia's Talk page:
I caught Joey's Rattata with the "catch trainer's Pokémon" Gameshark cheat, and it really does have all-zero IVs. It also has a Gentle nature (+Sp.Def, -Def).
If what they say is true, then the rattata is not "in the top percentage". However, I will add that the nature of trainer's Pokemon are probably random, so that part probably shouldn't be counted.