Dragonite can effectively of use either or, depending on what Set he is running. For a Physical Set, the most common type of Dragonite Set, Brick Break is typically the superior option. However, if he is running a Special Set, which is commonly seen on Special Rain-Knight Sets, Focus Blast is the better option. However, you can also run a Mixed Set, in which Dragonite can use the higher powered option (Focus Blast) to deal more damage to Physical Walls that would otherwise sponge a Brick Break well. Below are the mentioned Sets, the first being suggested the most:
Physical Set
Dragonite (F) @ Lum Berry
Trait: Multiscale
EVs: 252 Atk / 252 Spd
Naive Nature (+Spd, -SDef)
- Dragon Claw
- Brick Break
- ExtremeSpeed
- Dragon Dance
Physical Dragonite is the most standard Set that it can run, and for a great reason. With massive Attack Stat, Dragonite can naturally fire off "extremely" ExtremeSpeed s that other Pokemon would have to equip a Life Orb for, and can fix up it's middling Speed with Dragon Dance, making it even more powerful until eventually it's GG. While Dragon Claw is it's most powerful Attack as it's STAB, Brick Break is there to remove the opponents that resist it. The given EV Spread is pretty straightforward.
Special (Rain) Set
Dragonite (F) @ Leftovers
Trait: Multiscale
EVs: 252 HP / 4 Def / 252 SAtk
Modest Nature (+SAtk, -Atk)
- Hurricane
- Focus Blast
- Thunder Wave
- Roost
An underused Set that more than deserves mention due to it's effectiveness under the Rain. Hurricane is the STAB of death, while Focus BLast is there to remove the that Pokemon that resist it. Thunder Wave slows down it's foes so he can make great use of Multiscale with Roost. The given Set is pretty straightforward.
Mixed Set
Dragonite (F) @ Lum Berry
Trait: Multiscale
EVs: 204 Atk / 52 SAtk / 252 Spd
Naive Nature (+Spd, -SDef)
- Dragon Claw
- Focus Blast
- ExtremeSpeed
- Dragon Dance
A Mix between both the Physical and Special Set. He has the same idea as both, except he now has the ability to attack from both sides of the spectrum. The given set is also pretty straightforward.
Hope I helped!