Not completely sure what you mean by "support" here; support Pokemon are ones that set up Stealth Rock, Rapid Spin or have Wish/ Heal Bell, like Skamory, Blissey, Donphan, etc.
It would also depend on which tier you're in. Flygon for UU and Salamence for OU.
Flygon can only be a physical sweeper, while Salamence has the option of being Special/Physical/Mixed.
Both of these dragons have a x4 Ice Weakness and immunity to Ground, but Salamence has an added x2 Rock weakness and Stealth Rock will cut its health by 1/4. Flygon has a Rock resistance, which is useful on a Stealth Rock switch in.
Flygon @ Choice Scarf
EVs: 252 Atk, 252 Speed
- Fire Punch
Flygon will make a good late game sweeper, being able to outspeed most things when Scarfed, and clean up Pokemon like Starmie with a U-Turn. It will also be able to deal with Scizor with Fire Punch.
Salamence @ Life Orb
EVs: 252 Atk, 252 Speed, 4. Sp. Atk
- Dragon Dance
- Outrage/ Dragon Claw
- Fire Blast
Salamence can learn Dragon Dance, which Flygon can't, meaning it can boost its attack and speed without being choiced.
Both Pokemon will be cleared away with one non-STAB Ice Beam. A Porygon2 with no Sp. Atk investment will be able to clear Salamence/ Flygon.
Neither should be used for support. Flygon is best used for UU late game sweeping (also counter sweep because of U-Turn), Salamence as an OU/ Uber Moxie sweeper.