Mightyena for sure. Not only my favourite Hoenn 'Mon, but my favourite overall 'Mon, AND my first ever shiny about 9 years ago who I still have today (in OR, so he finally returned to Hoenn after all these years).
Found him as a Shiny Poochy just outside Fortree City in the tall grass (tried to replicate it in OR, but realised Poochy was exclusively found there in Emerald). Was absolutely ecstatic since I'd never seen a shiny before and it was my favourite 'Mon. Caught the lil' guy in a Great Ball and named him GOLDWONDER (yes, all caps too - I was 10 years old haha) and being able to see him in Amie all these years later was just the best thing.
He's pretty much the main reason why other shinies nowadays have zero value to me - they're far too easy to encounter and I've got no personal attachment to any of them (except one from last year: an unexpected Absol that hatched as soon as we brought my dog back home, after she'd escaped the garden that morning (so I named the shiny Absol after her)). The only other shiny even mildly worth anything to me was from FireRed a couple months ago: I was doing a nuzlocke and genuinely came across a shiny male Nidoran within ten minutes of catching my first encounter (a regular male Nido). Needless to say, it'll make a great comic once I get around to it. :P
Also, your signature is hypnotising.