Gyarados and Floazel both have more physical attack stat. They both want physical attacks.
These are probably their best final movesets, assuming you're okay with using TMs.
- ice fang
- earthquake/stone edge
- dragon dance
- ice punch
- brick break/crunch
- rain dance
If you already have one of them and already started giving it experience, then you should just keep that one. If you don't, then it depends on which opponents you care about more. Gyarados has intimidate and is better against Volkner and Aaron. Floatzel is better against the rival, Fantina, Candice, and Lucian. If you decide to use Gyarados, then you should use a good rod to get a Magikarp at least level 19, so it evolves faster.