Introduced in Ruby and Sapphire, Flygon was a new Dragon-type Pokemon that proved very useful.
Generation III saw several new additions to the Dragon-type Pokemon category. Out of them all, Flygon may be the easiest to obtain with Trapinch, then evolving into Vibrava, and finally into Flygon.
With a solid balance of stats, Flygon is very fast and can be used in a variety of situations. It has a slightly higher base Attack stat, but its Special Attack and defensive stats are not too far behind.
Note: This article is subjective and reflects the opinion of the writer.
The best moveset for Flygon in Pokemon Ruby and Sapphire
As a Ground/Dragon-type, Earthquake will be Flygon's primary move in battle. In Pokemon Ruby and Sapphire, dominating opponents is key with a Dragon-type creature. Earthquake will receive a Same Type Attack Bonus and deal solid damage to anyone it goes up against.
Dragon Claw will give Flygon another Same Type Attack Bonus. It is a pretty powerful Dragon-type move that will help Flygon with any Dragon-type Pokemon it may end up against. 100% Accuracy, 80 Power, and just pure Physical damage makes this a wonderful attack for Flygon to learn via TM.
Crunch is a good coverage move that Flygon learns by simply leveling up. It will do supereffective damage against members of the Elite Four and will have no issue hurting other Pokemon that do not resist it. It may also lower the target's Special Defense stat in Generation III, which could set up a teammate very well.
The final attacking move for Flygon could be either Rock Slide or Flamethrower. Rock Slide can cause flinch and be learned via the Move Tutor, while Flamethrower can cause burn and be learned via TM.
This all depends on what the rest of the team looks like. If you have another solid Fire-type Pokemon or Fire-type attack user, go with Rock Slide. Both will help with Ice-types that are dangerous to Flygon.