The very first time that I found what shiny Azumarill looked like, I fell in love with them. Something about it being so strikingly different from it's normal color scheme caught my eye, and if I were to ever get one of my own, it'd probably be my favorite Pokemon of all time (sorry Butterfree!).
As for the ones I don't like, there are quite a few off the top of my head (Gengar, Zapdos, Elekid, Garchomp, Lillipup, etc.) that are just a few shades darker/lighter, making their shiny forms a bit dull. At least with something drastic like a bright yellow Azumarill or a lime green Zubat, you know right away that it's special.