Powertechs make the best tanks due to their hybrid spec between Tank and Prototype. They have the best tankiness because they have the highest straight damage reduction, mines at 54%, a similarly geared shadows is 42% tops.
Assassin is a better hybrid, they do more damage, their dcd is much better, they have more mobility, better cc's, and can cloak(guard nodes better).
Juggernaut has better cc's, better mobility, and can darn near perma slow opponents. They also have better dcd's and because of their area snares, can really improve aoe dps on groups.
Sorry, but powertech is the worst pvp tank. They have no where near the dps or usefulness of the above.