Choosing a Ribbon Master
Picking a Pokémon for your Ribbon Master is one of the most important things about this challenge as this will be your trophy Pokémon that you will be with for the duration of the challenge.
There is a list of species of Pokémon that can get every possible ribbon (which I will explain below), but I will recommend that you choose a Pokémon that you like instead, since the list of species that can get all ribbons is steadily decreasing due to modern games cutting available Pokémon.
There are 4 sets of ribbons that are not obtainable by certain Pokémon that are caught in gen 3:
Battle Tower Ribbons: Restricted legendries cannot get most of these types of ribbon.
The Winning Ribbon: Pokémon that are over lv50 in gen 3 cannot get this ribbon.
The National Ribbon: Only obtainable for Shadow Pokémon from the GameCube games.
Restricted Pokédex Games: Pokémon that are not available in the following games will not be able to get ribbons that are unique to these games.
Sword/Shield, Brilliant Diamond/Shining Pearl, Legends: Arceus, Scarlet/Violet
If your chosen Pokémon is ineligible for any of the above ribbons feel free to skip over any sections of the guide that don't apply to them. A Pokémon is still a Ribbon Master as long as it gets every ribbon available to their species (i.e. Eevee cannot get the National Ribbon, so it can skip that ribbon)
With the above in mind, below is a list of Pokémon that can get all obtainable Ribbons (and you can find where to catch them here):
XD: Gale of Darkness
Colosseum (Scizor is an E-reader exclusive)
If your Shadow Pokemon comes from Colosseum, it also has the potential to be shiny (all Shadow Pokemon are shiny locked in XD). Be warned that hunting for a shiny shadow in Colosseum is VERY HARD and has a lot of unusual quirks compared to a normal hunt, This Guide by Oliver_MKP explains the system well.