I think Salamence fits the bill the best.
Good Attack & Special Attack
Hydreigon has higher stats than Haxorus across the board with the exception of Attack. It has 105/125/98 offenses while Haxorus is more physically inclined with 147/60/97. Salamance, however, has even better balanced stats than Hydreigon with 135/120/100, making him the fastest and most well rounded.
Hydreigon gets Crunch, Dragon Pulse, Surf, Fly, Earthquake, Return, Flamethrower, Fire Blast, Charge Beam, Stone Edge, Rock Slide, U-Turn, and Flash Cannon. It also gets Earth Power, Heat Wave, Outrage, Dark Pulse, Aqua Tail, Roost, Superpower, Draco Meteor and Zen Headbutt via Move Tutor.
Haxorus gets Dragon Claw, Outrage, Surf, Earthquake, Brick Break, Aerial Ace, Shadow Claw, Rock Slide, X-Scissor and Poison Jab. It also gets Aqua Tail, Low Kick, Draco Meteor and Superpower via Move Tutor.
Salamence gets Fire Fang, Thunder Fang, Crunch, Dragon Claw, Dragon Pulse, Zen Headbutt, Fly, Earthquake, Brick Break, Flamethrower, Fire Blast, Shadow Claw, Stone Edge, and Rock Slide. It also gets Draco Meteor, Aqua Tail, Heat Wave, and Roost via Move Tutor.
Winner: I think Hydreigon wins this one, but Salamence isn't too bad either.
While both Hydra and Haxy have decent Speed stats, Hydreigon is faster by one point (this can vary with Natures, EVs and IVs however). Salamence is the fastest with 100 base speed, letting outspend pretty much anything the game can throw at you.
Winner: Salamence, but remember Haxorus can boost with Dragon Dance.
Helps beat the Champion
Salamence's better stats, bulk, and balanced offenses make it a good trump card for Iris, especially if you can move before her Dragons. Hydreigon has a slightly better move pool, though.
Winner: Probably Salamence.
Have access to Swords Dance or Dragon Dance
This is where Hydreigon and Salamence lose. While Haxorus gets both D-Dance and Swords Dance, Hydreigon and Salamence get neither. However, Hydreigon does get Work Up to boost both its Attack and Special Attack.