So let's start by evaluating each Pokemon!
Goodra
- Has 3 weaknesses and 4 resistances (Sap Sipper makes 3 resistances and a Grass immunity)
- Really good SpA, 110 is great
- Has an insane 150 Special Defense, and an okay 70 Defense, as well as good HP, 90
- Has access to both Ice Beam and Blizzard
- Only main drawback is base 80 Speed, maxes out at 284. Still okay, however!
Summary: Bulky Special Attacker, maybe lacking speed.
Greninja
- 5 weaknesses, 6 resistances, and a Psychic immunity
- Good speed
- 103 SpA which is good
- Defensive stats are 72/67/71, has to outspeed as it's quite frail
- Has Ice Beam, Icy Wind, and Blizzard
- Great Abilities in Protean (changes type to get STAB for every move), and Battle Bond, Ash Greninja is just so good, after a knockout, turns into Greninja-Ash and Water Shuriken always hits 5 times.
Summary: Pretty "eh" Defensive stats, but good Speed and SpA, as well as a wide Special movepool (+ Ice Beam/Blizzard/Icy Wind). Good!
Dracovish
- 2 4x resistances, a 2x resistance, and 2 weaknesses
- Max 273 Speed, but Choice Scarf boosts that
- Massive potential with Fishious Rend, eats teams up
- Only Ice move is Ice Fang (although it can be boosted by Strong Jaw)
Summary: Dracovish has a great Physical moveset and is an incredible Physical sweeper with Fishious Rend.
Who should you pick?
I would choose Goodra (just my opinion though)
Here's my best set, you don't have to use it (I'm not amazing at competitive), but it's my best set.
Goodra @ Life Orb / Choise Scarf
Ability: Any
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Dragon Pulse
- Ice Beam
- Thunderbolt / Sludge Bomb / Focus Blast / Flamethrower / Hydro Pump
- Thunderbolt / Sludge Bomb / Focus Blast / Flamethrower / Hydro Pump
Dragon Pulse is STAB, Ice Beam is a Special Ice move. Any other Special move fits well with LO. Maxed Speed and SpA EVs help Goodra become a sweeper.
Greninja and Dracovish are also good Special/Physical sweepers respectively, you can choose whichever suits you playstyle more.
Hope I helped! :D