After you beat the Elite Four, you will be able to face off against the champion who is the most powerful trainer of their respective region. So hit them with everything you got!!!
Which Champion music do you like best?

Kanto (Blue): Red/Blue , FireRed/LeafGreen , Black2/White2

Johto (Lance/Red): Gold/Silver , HeartGold/SoulSilver , Black2/White2

Hoenn (Steven/Wallace): Ruby/Sapphire , Black2/White2 , OmegaRuby/AlphaSapphire

Sinnoh(Cynthia): Diamond/Pearl , Black/White , Black2/White2

Unova(Alder/Iris): Black/White(Alder) , Black2/White2(Iris)

Kalos(Diantha): X/Y