I remember this same topic coming up years ago in the 3rd generation. At least half the responses said Hoothoot in G/S or Whismur in R/S, which at the time were also the only 2 shinies I'd ever got (other than Red Gyarados). I wonder if those 2 have a higher likelihood or something.
Cranidos, which I got in platinum after I had basically finished the game. Easiest shiny to get, and probably the most shocking. *give fossil to guy, go outside then back in, get Pokemon back* Yay. Unless you're counting the Gyarados, then that. Although I doubt you're counting that :P