Growlithe, on a chain of 26. It broke soon after, so I was lucky! I didn't really know how to use the Radar back then.
I have the feeling, and vaguely recall, that before I knew about shiny Pokemon I'd run into one or two, thought "Oh, the game's glitched up!" and left the battle, heh. Oh well, you live and learn. My cousin once ran into a shiny Tentacool and had no idea it was shiny. Needless to say, he KO'd it.
Now I have all sorts of shinies from the PokeRadar, and a couple of hacked ones which I intend to release. They were just to complete the 'dex. But I digress.
If anybody would like to trade for one of the shiny Phanpy that I chained yesterday, PM me. Only legit shinies in return though ;)