Do you have Pokémon Ruby or Sapphire? Rayquaza is located in the Tower of Heaven and getting there is pretty straightforward.
Steps

Step 1. Beat the Elite Four

Step 2. Swap your bike in the bike shop in Ciclamipoli for a Bici Corsa (if you don't already have one)

Step 3. Fly to Orocea

Step 4. Proceed to the right from the Pokémon Center
Use Surf until you reach a red-haired (moving) swimmer. At this point, go up, then left. You will find a cave.

Step 5. Go through the cave to reach the Tower of Heaven

Step 6. Enter the tower

Step 7. You need the Race Bike to get through the puzzle
- Quickly pass over the cracked floor spots. It is better to study in advance the path to take in the maze (by reading a guide on the internet or with experience).
- On the second floor with the cracked floor, proceed to the end of the maze and drop through the third to last square. You will reach the stairs blocked by a stone. This is the only complex passage, for the rest you just need to go up the stairs.