I was going to use images, but don't have enough posts =_=
Manectric: it turns black and yellow, and looks awesome.
Zangoose: the blue markings seem to look better than the red, to me.
Absol: Well, it just looks cool when it's red, I suppose. Goes well with the white, too.
Salamence: It's a traditional green dragon. What is there to not like about it?
Dugtrio: really, blue noses? They have bodies, for petes sake, yet it's the nose that changes...
And of course, those other two shinies which everyone and the cat loves... >_>