Well it really depends on what kind of dragonite you want. If you want to raise a special attacking dragonite then rash, modest (eventually quit(not recommended) will all benefit it's special attacking stat. Dragonite's special attack can reach 328 stat maximum. But adamant is usually the preferred nature for dragonite. It raises it's wonderful attack stat. In contrary to dragonite's special attack stat, an adamant dragonite's attack stat can reach 403 if it is flawlessly trainend. Personally I would recommend an adamant natured dragonite, but the choice is yours. It really depends what kind of dragonite you want and in what kind of team you need it for.
Hope this helped and good luck