The most notable thing on Zweilous is hustle which powers up physical moves by 50% at the cost of 20% accuracy. This make it the strongest outrage in the game bar Kyurem-Z. It can also use Eviolite to boost up his defense. His low 58 Speed allows many opponents to KO Zweilous before he can dish out the Outrage though. The accuracy lost by Hustle can be game changing too so he is very risky to play. His movepool straight out sucks. He has the elemental fangs, Outrage, Crunch, Dragon Tail, and that's about it. So he all or nothing really.
High attack paired with Dragon Dance and Outrage is dangerous. As with Zweilous, he can use an Eviolite boost up his defenses but Fraxure has less bulk to take advantage of the boost. His 68 Speed isn't very great although it can be boosted by Dragon Dance. He also has Swords Dance to double his attack and can even run a double dance set. His movepool isn't too great so it doesn't allow very many options. Eviolite is the item of choice but a Life Orb can be used but more power. Mold Breaker is the better ability here as Rivalry may screw you up. Reliability-wise, he is better than Zweilous but Zweilous has better bulk and straight away power.
Stupid looking guy, he comes with a higher base attack than Fraxure and a better movepool to play around with. It sadly cannot use Eviolite, which is fine because he has a good 77/90/90 bulk. 48 Speed really sucks and will be out-sped by many other pokemon but he has good bulk to compensate. If pulled off, Trick Room can make him a formidable opponent. Druddigon can use Leftovers to take more hits while a Choice Band and Life Orb will give more power. As for abilities, Sheer Force can only really power up are Fire Fang, Thunder Fang, and Crunch. He can become a very good tank that can hit back hard.
This is a tough choice. As for a simple boost up sweeper, Fraxure get the job done. Zweilous is risky because the accuracy drop may screw you up although if you hit the Outrage, it will KO something that doesn't resist. Druddigon can tank better with Leftovers than Zweilous and can dish out well with his better movepool although lacking the raw power from Hustle. Pick whoever you think can fit your team best.