I fondly remember a good old Poliwag from my Red version. I traded it to my Gold after evolving it into a Poliwrath and teaching it Fissure via TM so I could teach it Mind Reader. After trading it, I was shocked to see it was shiny (which, unless I'm mistaken, meant perfect IVs back then)! Sadly, my Gold version's battery has died a long time ago, taking my shiny haxxing Poliwrath from me.
Right now, my special Pokémon is my shiny Venomoth, which I got off the GTS. It didn't have perfect IVs or nature, so that leads me to believe it isn't hacked (and I wouldn't care much if it was, since I don't play competitively or plan to do so). Venomoth was my favourite Pokémon since Red, and I was really happy when I managed to get my hands on a shiny one.
A honorary mention goes to my Red version Lv. 16 Sandshrew, which taught me why Sand Attack was useful by single-handedly defeating the fighting dojo's Lv. 30 Hitmonchan when the rest of my party had fainted.