If this is ORAS, I recommend Thunder Punch, as it counters Water type using Ice Beam on your Garchomp. If it's X/Y, STAB Stone Edge is always nice. In SM/USUM, I recommend, again, Thunder Punch, though Aqua Tail/Ice Punch for Ground types is also good. Another option is Rock Polish for boosting, or Fire Punch for and Ice types who try to use STAB Ice moves onto Garchomp.
Oops I forgot Superpower in USUM/ORAS. That could also fit in there.
Hope this helps! :)