Gyarados and Floazel both have higher physical attack stats, but ice beam has higher base power than ice punch and ice fang. According to the damage calculator, Floatzel's 2 attack stats are close enough for ice beam to do more damage than its physical ice moves when the opponent has equal defenses.
These are probably their best final movesets, assuming you're okay with using TMs.
- ice fang
- earthquake/stone edge
- dragon dance
- ice beam
- 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.