Here are a few useful notes before you start the challenge
Make sure to get the Winning Ribbon in Gen 3 before you get to LV 51, as you CANNOT GET THIS RIBBON WHEN YOU ARE ABOVE LV 50.
Ribbons.guide by u/10Sly10 is a great resource that allows you to keep track of your progress for multiple ribbon masters
If you are interested in figuring out your Pokémon's IV’s or Hidden Power, you can follow the links.
If you are unsure of moves, items, etc. that are good on your Pokémon in any generation, Smogon is a good starting point (even if it is designed with PVP in mind)
r/pokemonribbons is a subreddit dedicated specifically to Pokémon ribbons, and is a great resource if you have questions about any aspect of the challenge.
The order that you obtain ribbons is not super important in most generations but I have a recommendation for the sequence in Gen 3 and early Gen 4 in order to make the run as smooth as possible.
National Ribbon, as you cant transfer to GBA without it
EV training for your Effort Ribbon in R/S/E
If your ribbon master is under lv50, attempt the Earth Ribbon until your Pokémon is lv50 (do not let it go above 50), Mt. Battle is a great source of exp. and is the way to PokéCoupons to get rare berries for step 7.
Complete the lv50 Battle Tower for the Winning Ribbon.
Finish the Earth Ribbon if you didn’t already in step 3.
Level your Pokémon to lv100 (if doing the battle tower on Ruby/Sapphire) or lv60 (if doing the battle tower on Emerald). If you still need PokéCoupons then you can both gain exp. and earn coupons by repeating Battles 91-100 in Mt. Battle.
Start planting/obtaining the berries that you need to turn into PokéBlocks so will have enough when we get up to contests (for recommendations on berry choice go-to Contest Ribbons in the Hoenn section of this guide)
Obtain both the Champion Ribbon and the Victory Ribbon in any order
Create and feed PokéBlocks, and change your moves to contest oriented ones, then get all 21 Contest Ribbons
Transfer your Pokémon to Gen 4, then obtain all 20 Contest Ribbons
Swap your moves back to battle oriented ones, then obtain all the other ribbons in any sequence you want.
(this information applies to all variations of Battle Tower, Maison, Tree, etc.)
Acquiring Ribbons
In order to obtain a Battle Tower Ribbon your Pokémon does not need to be part of the entire streak, instead they can just be a part of the final set.
For example in Emerald you get the ribbon at 56 Battles and they are done in sets of 7 Battles, this means that you can run any team you want for Battles 1-49, and for Battles 50-56 you bring your Ribbon Master.
This is a very useful strategy if your chosen Pokémon is weak in battle.
Trainer Sets
For each game that has a Battle Tower section, I will provide links to sets of Pokémon that trainers you encounter will use.
Each trainer has a large list of Pokémon that they can use, this would not seem very useful at a first glance since the pool they can pick from is large but most trainers have only one copy of each Pokémon, so for example if you see that a trainer sends out a Pikachu and that trainer only has one Pikachu on its list you know that their Pikachu is going to be exactly what's on the list.
This can be very useful information as it can tip you off to dangerous moves for your team.
Team Building
There are a massive amount of strategies when it comes to team building in Pokémon and no single strategy is the best, if you want to make your own team that's totally fine!
I found success with a team built in this way:
A fast lead Pokémon with good type coverage to clear out the majority of what you face.
An offensive Pokémon that can take a hit and retaliate, ideally its type compliments the lead.
A defensive Pokémon that can win by setting up and sweeping, or whittling down troublesome targets with status.
I recommend making a really solid team in generation 3 that you can use in future generations as well, this means that you wont need to breed and train a new team every generation, and can just get straight into clearing the tower. Below is a sample team that can can do this, if you want to use your ribbon master, they can replace one of the options below that is most similar to the ribbon master:
Latios (Timid) (EVs: 4 HP / 252 SpA / 252 Spe) (Lum Berry/Life Orb)
Psychic
Dragon Claw / Dragon Pulse
Thunderbolt
Ice Beam / Calm Mind
Metagross (Adamant) (EVs: 4 HP / 252 Atk / 252 Spe) (Choice Band/Assault Vest)
Earthquake
Meteor Mash
Shadow Ball / Bullet Punch
Explosion / Zen Headbutt
Suicune (Bold) (EVs: 252 HP / 252 Def / 4 SpD) (Leftovers)
Surf / Scald
Calm Mind
Rest
Sleep Talk