Timid or modest for this bitch?

depends on the set.

Timid can work with Shift Gear. Modest has more general utility. If you run Timid, you basically have to always use Shift Gear or the nature will be wasted, so it's more restrictive. But if that's what you wanna do, then go for it, it can work.

If you want to use Shift Gear just do modest with enough speed EVs to outpace full speed Pheromosa and SpA. Dump the rest in HP.

Timid might br OK too. Are there any relevant outspeeds you can with a +Speed nature only?

>>30666268
Scarf Garchomp I believe can be outsped with Timid, Max Speed, and Shift Gear.

Modest if you're not a gamblin' man.

But,
If you are a gamblin' man.
Go timid.