They both have base 600 stats, and practicallly opposite roles.
Tyrannitar can be used at a weather setter and physical sweeper, and I sometimes like to trip people up by using Specs T-Tar with Unnerve in doubles. It has bulk, low spd without investment, and it generally used to tank anything in its way.
It has a 4x weakness to fighting, which can be remedied with Chople Berry. It is also weak to Ground, Water, Grass, and Steel, and so needs coverage from team members.
Hydriegon has high spd, high special attack, and not much else. It is commonly seen on clear Skies teams as opposed to weather, and it used as a Special/mixed Sweeper. It can KO many Pokemon easily, but won't be able to take most STAB SE hits.
It is weak to Ice, Dragon, Fighting, and Bug.
As you can see, they have completely different roles, and shouldn't be compared to see who is "best".