Since both of these Pokemon are psuedo-legendaries, they have very high base stat totals for in-game (600).
I won't factor in availability, so feel free to let me know after if availability should be accounted for.
Stat Spread:
Both of these Pokemon have the same base stat total, so we'll be looking at their stat spread.
Salamence has an incredibly high base Attack stat of 135, and its Special Attack isn't that bad either, at 110. Its Speed is good for in-game, so it becomes quite a good hyper offense Pokemon. Its defenses aren't too bad, sitting at a decent 80. Overall, solid stat spread.
Goodra is fit more to be a bulky attacker. Its Special attack is the same as Salamence's, which is alright for in-game. Its attack stat is also alright, sitting at 100. This, however, means that Salamence has higher offenses than Goodra. Although its offenses are lower, the thing that catches your eye is that insane Special Defense, sitting at 150. This makes it more of a Special wall rather than an attacker, so it seems that Salamence is winning because in-game teams value hyper-offense more than stall/walls.
Typing:
Salamence typing is Dragon/Flying. This gives it 5 resitances and an immunity, whilst also giving it 4 weaknesses, one of them being a 4x weakness to Ice. Offensively, however, its not really good. It hits 4 types for SE damage, one of them being Bug, one of the worst types in the game. This could be fixed by Terastallizing, maybe Tera Fire or Fighting to take care of the Ice/Steel types, or using coverage moves (more on that later). An advantage of having the Dragon type is that you hit many Pokemon for neutral damage, and the 2 types that you don't, just use coverage moves.
Goodra is a pure Dragon type. This gives it 4 resistances and 3 weaknesses. Offensively, this type is pretty bad. Hitting a measly 1 type for SE damage. As said above, you hit many Pokemon for neutral damage, but hitting more than 1 type for SE damage would have been nice. It does have 1 less weakness than Salamence, while also not having the 4x that Salamence has, but it comes at the cost of 1 resistance and an immunity. Terastallizing can fix that also, maybe Tera Steel to make its weaknesses turn into resistances. Overall, Salamence has the better typing.
(Note that if you are using Sap Sipper on Goodra, it has 3 resistances and 1 immunity instead.)
Ability:
Salamence has the ability Intimidate, which is quite a good ability. Moxie is also a good ability, but its a hidden ability and it's tedious to grind for a HA one. Intimidate helps a lot with physical attackers, and it also lets Salamence set up Dragon Dance on physical attackers (More on DDance later). Nothing much to talk about. Overall solid ability.
Goodra has Sap Sipper and Hydration. Sap Sipper is an alright ability, allowing Goodra to switch in against Grass types and gain an attack boost. It might not be better than Intimidate, but its an alright ability. It makes Goodra's resistances go from 4 to 3 and gives it an immunity. Hydration is situaitional and shouldn't really be used. Overall, alright ability.
Moves:
Both of these Pokemon have a wide range of useful moves. Salamence has access to Dragon Claw and Fly/Dual Wingbeat for STAB, and Rock Slide, Earthquake, Zen Headbutt, Crunch and Iron head for coverage on the physical side. On the special side, it gets Dragon Pulse and Air Slash/Hurricane for STAB and Hydro Pump and Flamethrower for coverage. But most importantly, it gets Dragon Dance, one of the best set up moves in the game. Along with Intimidate, it can set up on physical attackers and sweep entire teams. Overall, excellent moves.
Goodra gets a good amount of coverage moves, getting Dragon Pulse for STAB, and Thunderbolt, Ice beam, Flamethrower, Sludge Bomb and Hydro Pump on the special side. On the physical side, it gets Dragon Claw for STAB, and Rock Slide, Earthquake, Fire Punch and Thunder Punch for coverage. It doesn't get Dragon Dance or anything of the equivalent, so it doesn't have the sweeping potential that Salamence has. Overall, solid moves.
Something also worth noting is that Sliggoo is a pain to evolve. You need rain in the overworld, which the player cannot control, so thats there.
In conclusion, Salamence is superior to Goodra in quite a few ways for in-game teams. It has very good sweeping potential, and a good ability, which is excellent for in-game teams.
Moveset:
EVs are optional.
Salamence @ Sitrus Berry
Ability: Intimidate
Tera Type: Flying
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Dragon Dance
- Dragon Claw
- Earthquake
- Fly
Dragon Breath ---> Dragon Claw
Bulldoze ---> Earthquake
Fly is obtained upon evolution into Salamence
Dragon Dance can be obtained in North Province Area One.
If you really want to use Goodra despite this answer, then here is a moveset.
Goodra @ Sitrus Berry
Ability: Sap Sipper
Tera Type: Steel
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Dragon Pulse
- Thunderbolt
- Ice Beam
- Flamethrower
Dragon Breath ---> Dragon Pulse
Its coverage is obtained quite late, so you can use moves like Water Pulse, Mud shot etc.
Hope this helped.
