Dragonite works well on a rain team having different weaknesses to the commonly used Water type Pokemon and - even though it's attack stat is higher, at 134. It has a moveset that could accompany it well as a special sweeper.
Multiscale and Leftovers is a good combo as Dragonite can hold out against powerful attacks long enough to sweep a few Pokemon. Also, some people may assume that your Dragonite, like many others, is physically offensive due to it's attack stat being highly superior and so send out a Pokemon with high Physical Defense not Special Defense allowing for easier sweeps.
Dragonite @ Leftovers
Ability: Multiscale
EVs: 252 SpA / 128 SpD / 128 Def
Modest Nature
- Thunder
- Hurricane
- Draco Meteor
- Focus Blast
Thunder: Powerful electric attack with full accuracy in rain
Hurricane: Powerful STAB flying attack with full accuracy in rain
Draco Meteor: Powerful STAB dragon type move
Focus Blast: Powerful fighting move that is super effective against ice types
Dragonite's best stat is it's attack by far which is supported by numerous, Physical, moves - including those with STAB. In fact, a Dragonite could function well on a Sun team.
Dragonite @ Lum Berry
Ability: Multiscale
EVs: 252 Atk / 128 SpD / 128 Def
Adamant Nature
- Dragon Dance
- Outrage
- Thunder Punch
- Fire Punch / Rock Slide
Dragon Dance: Increases Attack and Speed to set-up for a sweep
Outrage: Powerful STAB dragon type move
Thunder Punch: Gives a Sun team variety and covers it's likely water weakness (as a team overall: mostly fire types)
Fire Punch / Rock slide: Super effective against Dragonite's weakness to ice and either has more power - because of sunny day - or hits both opponents in a double battle as well as giving the team variety concerning it's overall moveset.