Colosseum & XD: Gale of Darkness are amazing games and they introduced players to a brand new region called 'Orre' these were the first Pokemon games not to have Gyms and you had to catch Shadow Pokemon and then purify them.
The games had limited Pokemon (48 Shadow Pokemon in Colosseum and 83 in XD: Gale of Darkness) so you had to build a team around the Pokemon available to you some were easy to purify and others were hard to purify.
This was my team on Colosseum from my original playthrough in 2004
I really loved Mount Battle its the perfect place in both Colosseum & XD: Gale of Darkness) to train your Pokemon and if you defeat all 100 trainers in a row without changing your team or leaving Mount Battle you are rewarded with the Earth Ribbon and if you are playing XD: Gale of Darkness you get a choice of Johto starters Chikorita, Cyndaquil or Totodile with their elemental Hyper Beam (Frenzy Plant, Blast Burn or Hydro Cannon) as well as the Earth Ribbon.
The process to purify Shadow Pokemon was so much better in XD: Gale of Darkness as it introduced the Purification Chamber and it became much easier to purify Shadow Pokemon.
Realgam Tower in Colosseum felt like the Orre region's version of the Pokemon League with 6 battles to take down Evice the boss of Cipher that battle is the hardest battle in the entire franchise until BDSP in Gen 8 and Cynthia's 3 Pokemon League battles.
Ciatdark Isle in XD: Gale of Darkness gives you such a creepy atmosphere and I love the music on this area I was very overlevelled when I first got to Citadark Isle due to the amount of training I had done on Mount Battle.
I consider Colosseum & XD: Gale of Darkness main series games due to the fact that you can transfer the Pokemon you catch in Colosseum & XD: Gale of Darkness to Ruby, Sapphire , Emerald, FireRed & LeafGreen.
What did you enjoy about Colosseum & XD: Gale of Darkness.