Gyarados has two useful abilities, Moxie and Intimidate. It also has Dragon Dance, a great move for sweeping. It is also a good defensive Pokemon, and it can easily take some hits from Greninja and Swampert. It also has access to mega evolution, which make it more powerful. It can learn Waterfall, Earthquake, and many other good moves.
You may wish to use Cloyster for Skill Link and Shell Smash, but I think Gyarados is more useful.