Re: Surf or Hydro Pump?
Surf is the more practical choice in-game, because not only can you use it as a HM on an essential Water-type team member (very convenient, trust me), you have more PP as a Water attack. In-game battles aren't normally ridden with Pokemon who are EV-trained, so you can normally use Surf to take them out, especially those that are weak since this move has a lot of power and always hit. Hydro Pump is not as practical, because it is not as accurate and 5PP isn't very conducive in-game, even with PP Ups, since you need to restock your PP in the most inconvenient ways. However, Hydro Pump is better in player VS player battles because it puts offensive pressure on the opponent, so it's not surprising to see opponents like Starmie and Kingdra prefer this move, especially on a Rain team.
Thanks for reading.