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in-game probably donphan, for netbattle, gyarados. I don't like donphan's speed and weakness to special attacks, and gyarados is better for attack and has a smaller movepool for that as its best move for HP flying is hard to get especially for the desired base power. If with desired stats, it can be awesome.
In my experience, Gyarados has too poor a sp.attk stat to pull off the water moves it learns, such as hydro pump or surf, despite the fact that these are STABed. As an in-game pokémon, I've always found donphan more reliable and effective than gyarados. However, gyarados is relatively good in competitive battle as a dragondancer seeing as it has a great attack stat. For water purposes though, you can do better.
Donphan has good attack and can use Earthquake to full power. But since there are a lot of Pokemon that are water type or know water moves, Donphan may not be the brightest choice. If I may ask, why do you need our suggestions?
No. Gyarados and Donphan are both physical attackers. Gyarados's special attack stat isn't even close to good (the highest it can possibly get is 240).
As for who's better, it depends. As said, Gyarados isn't so great ingame, while it's excellent competitively. Although Donphan's awesome in both areas. Meh, I guess I'll go with Donphan, although Gyarados is better than it in competitive, at least Donphan is good in both areas.
I used that same Gyarados in FR, coincidentally. Answering the question, I think the better one is Gyarados indeed. That set chaos provided (and which I also used in FR) is something the A.I. will never get by.