My favorite competitive mon. Let's get into it.
ADV Tyranitar was pretty much the best mon, for power and sheer versatility. In ADV, you switch in Flygon, you risk that it may be a special/mixed variant with Ice Beam. Send in Claydol EVed to take a few Crunches and you find out it has DD. Send in Skarm or Pert to Phaze it out and you get hit by CB Focus Punch....to beat Tar in ADV you had to learn its set first and you were guaranteed to lose something learning it. Dugtrio was the closest thing to a true check, because it couldn't switch in and now all T-Tars are EVed to survive CB EQ from Dugtrio....and Dugtrio can't get off a second cause it just died. Literally, this thing was the best. No mistake this thing had the highest ADV OU usage in both SPL and WCoP.....it's just so versatile and good at everything.
DPP it stayed versatile, but now it could abuse Physical Crunch and Pursuit, add a Choice Scarf to revenge kill Starmie, Rotom and Jirachi, three of the tier's top 5. Plus, with the addition of stuff like Stealth Rocks and bulkier Pokemon, Sand Stall became a lot more viable and effective, and Tyranitar was a lot more versatile than Hippowdon, and therefore saw a lot more usage.
BW it kinda fell from top dog status, due to not really getting a lot beyond BW2 Move Tutor Coverage options, while dealing with all the new Fighting Types and other threats: Lando-T, Terrakion, Keldeo, Conkeldurr, plus older ones like Breloom. However, it was still versatile, keeping all of it's DPP options: SR, Scarf and CB, and lastly Special Tank. Tyranitar also found 2 invaluable new niches: Checking the Lati Twins and stopping enemy weather, which were both extremely important, especially in BW2, where Rain Offense and 350 Speed LO/Specs Latios were running amok. Tyranitar trapped Latios and screwed weather based sweepers by turning off their speed and boosted attacks. This thing has been a dominant threat since it came out, solid in GSC, top dog in ADV and DPP, almost top dog in BW, and still extremely powerful in XY even.