For me, it was Dunsparce in the Dark Cave. I found it easy in Crystal when I was a kid, so when I went back in SS I spent hours trying to find it. Yeesh it took long... I really wanted that little snake in my starting team. XD
I WAS GOING TO POST THE SAME THING! I always had one on my team in Gold, Silver, and Crystal up until at least Chuck. Took me a lot longer to get one for HG/SS, and then I dtiched it for Kangaskhan in both too soon. ._.
EDIT javel34 posted...
I actually found a male one randomly in Ruby and failed at getting its beauty high enough to evolve, and then I kept it in the PC. Traded it to Emerald to Ditto-breed it, and I've just done that and transferred it up through the generations every time, a new one comes out. Never had the aggravation of searching for it like others.