Tyranitar, though is a lot more durable. Dragonite has lower Defenses and HP, and it has a weakness to Ice Beam, one of the most common moves in all of NetBattle. Fighting attacks aren't really that common, as they're quite useless without STAB. Most frequently you'll see them is on Heracross or Medicham =/ Otherwise, it's all in the clear for Tyranitar. It may have 6 weaknesses, but with such good Defenses and HP, it's very capable of resisting them.
Tyranitar also wins over Dragonite with DD. They both get it, but Tyranitar gets Taunt, which is essential to DDing. Dragonite doesn't. Meaning it can just get Whirlwind'd/Roar'd/Haze'd/Burn'd. Tyranitar stops all of that with Taunt, which is awesome. Even after one DD, not much stands up to Tyranitar. Give it a few more and you've just won

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Tyranitar is also better at SubPunching because of its base 100 HP. Dragonite doesn't get 101 Subs because its Base HP isn't high enough. Tyranitar, though, can SubPunch due to 101 Subs, which it can pull off with max HP EVs. Dragonite can do it, but Blissey just breaks through its Subs regardless. Tyranitar's Subs won't be broken due to Ice resistance if it has Ice Beam, and Seismic Toss resistance because of 101 Subs. Seismic Toss breaks through Dragonite's Subs, and Ice Beam does too because of that 4x weakness to Ice. Theoretically, Dragonite could Sub on the switch to Blissey and FP on the breaking of the Sub. But who would switch in a Blissey against a primarily Physical Pokémon anyway? =/
~*CB*~
The 8th Champion