Goodra
Pros:
- Very high special defense
- Extensive movepool
- Sap Sipper gives it a useful immunity
Cons:
- Offensive stats and speed are sub-par to it's competition
- Fairly frail physically
Garchomp
Pros:
- High attack stat
- Nice overall defenses
- Ground typing gives it a very powerful Earthquake
- Decent speed
Cons:
- Low special attack
- Dragon/Ground makes it 4x weak to ice
Salamence
Pros:
- Fantastic mixed attacking stats
- Decent speed
- Secondary Flying typing gives it a strong Fly
- Intimidate helps it tank hits better
- Access to Roost
Cons:
- Flying type makes it 4x weak to Ice and 2x weak to rock
- Lowest overall defenses of the bunch
So, now to deciding which one to use. I would rule out Goodra, as it has good defenses, but offense is the key in-game, and it's offenses are not up to par with Garchomp and Salamence's.
From here, both are good decisions, so I'm not gonna tell you which one to use. Use whatever one you want! Salamence is my personal pick, being able to abuse physical moves like Dragon Claw and Steel Wing while also being able to fire off powerful special moves like Fire Blast. But if you want to use Garchomp, give it a go!
