Ryu vs. Red
Red got the Pokémons he got in G/S ending. Ryu is at his strongest.
However, there is a rule:
Ryu will need to have defeated Red's Pokémons before he is allowed to kill Red.
All Red's Pokémons are close to level 100 (meaning max potential), and they are:
Pikachu
Espeon
Snorlax
Blastoise
Venusaur
Charizard
Red is allowed to use more than one at a time, but NO items.