I’m surprised this wasn’t already asked. Which ability is better for Garchomp? Sand Veil to boost evasiveness during sandstorm, or Rough Skin to deal damage when an opposing Pokémon makes contact? (I should mention that this Garchomp doesn’t have sandstorm in its moveset and neither do any of the other Pokémon in my party)
Sand Veil isn't very good. Smogon's Evasion Clause bans it entirely in some formats. Even if you're playing a format where it isn't banned, it's still bad, as your team has no sand setters whatsoever. Sandstorm is also generally a waste of a slot, so you would have to get a Pokemon that automatically sets it up.
Rough Skin is generally better. Those little bits of extra damage can make all the difference, plus it can make physical attackers think twice about hitting Garchomp.
Hope I helped!