There are tons of things that make it amazing.
A 50% boost to Rock types' SpD. Paired with his solid defenses, this made him very bulky. Weather used to be endless until replaced prior to gen 6. It provides support to teammates that like sand, such as Sand Rush users like Excadrill.
Very good 110 Defense and 100 HP, as well as 100 SpD that gets the Sandstorm SpD boost. 134 attack is also great. The only thing it lacks is it’s speed, which is a measly 61.
In gen 3, Tyranitar was blessed with Dragon Dance. This made his Attack and Speed good, which makes it a potent setup sweeper. It also has the amazing Stealth Rock, which paired with it’s bulk, makes it a good bulky rock setter. It’s got good STABs in Stone Edge and Crunch, as well as coverage in Earthquake, elemental punches, Dragon Claw, and more. It also has Foul Play and Pursuit, the former of which is awesome for a defensive Pokémon and the latter of which I will get to later.
In gen 6, Godzilla here got himself a mega. This gave him a massive 150 Defense, an amazing 120 Special Defense, and an extraordinary 164 Attack. It also gave him 10 more speed, which while not much, he still appreciates.
So many options!!!
Tyranitar is hard to predict. It can run so many sets. It can be mega, or non mega to hold an item, it can be a Stealth Rock setter, it could DDance in your face, it could tank every hit like a boss, you could accidentally trigger a 2+ attack boost from a Weakness Policy, you could have to deal with a huge hit from a Choice Band Tyranitar, it just has so many options!
Also, it’s niche in NatDex and Gen 8 Anything Goes is worth a mention.
It can kill the Bacon Bird and the Shadow Horse
Calyrex-Shadow and Yveltal have been running rampant recently. Yveltal is typically used to defeat Calyrex, but it doesn’t always succeed, as sometimes Calyrex escapes with Baton Pass or even delivers a big hit with a Z-Move or Dynamax. Tyranitar, with its massive Special Defense, walls Calyrex. Calyrex may have Energy Ball or something (although it’s uncommon), but Tyranitar can take it. You also can’t escape, because Pursuit will OHKO you before you can run away. Yveltal can only deal with Ttar if it has Focus Blast, which is rare. Even if the Yveltal Dynamaxes, it’s got no way around Godzill-I mean Tyranitar’s massive Special Defense. Tyranitar is a very niche pick, but it’s still got a niche!
Hope this helps!