I vote Ludicrousity (Why wouldn't I? It gives it a nice way to gain a last stand, and possibly recover all/most of it's health, and it's my submission)
The problem, and good thing, about Tyranitar is the fact that we need to make a Z move that has a niche over the mega, but not necessarily better, anyway...
Here we go!! :D
Z-Crystal Name: Tyranium-Z
Z-Move Name: Pursuing Strike
Z-Move Type: Dark
Z-Move Power: 180
Required Move: Pursuit
Z-Move Description: Tyranitar tracks it's target carefully and then lunges forward to strike. It cannot be avoided.
Z-Move Effect: Deals double damage to a protecting opponent, or an opponent that is switching.
Z-Crystal Name: Gigalithium-Z (Because Lithium)
Z-Move Name: Razor Rocks
Z-Move Type: Rock
Z-Move Power: 190
Required Move: Stealth Rock
Z-Move Description: Gigalith sharpens it's red rocks before charging at the foe. It leaves a rocky aftereffect.
Z-Move Effect: Deals 1/8 Max HP at the end of each turn and sets Stealth Rocks on the foes side of the field.
Z-Crystal Name: Hippownium-Z
Z-Move Name: Blinding Sandy Strike
Z-Move Type: Ground
Z-Move Power: 90
Required Move: Sand Tomb
Z-Move Description: Hippowdon releases a devastating burst of sand which swirls around the foe, trapping and blinding them.
Z-Move Effect: Deals 1/8 Max HP Per turn for 5-8 turns. Halves Accuracy and prevents switching. If the user is holding a binding band, the damage is changed to 1/6