Gyarados

I'm pretty sure it's either the best, or the second best (by a super close second) for attacking

For defending, I'm not sure

It's both. Hydro Pump is actually a pain to dodge because it comes with no warning (there is no text box saying "Gyarados used Hydro Pump!"). While the duration of hydro pump is 3.3 seconds, the damage is done at the 1.5 second mark, and most of the time you don't even know it's about to use the Hydro Pump, by the time you do, it's too late, and you take heavy damage. So I'd use that for both attack and defense.