Who cares about the party you join.
If the [filtered] game says Warriors ARE tanks and Warriors CAN tank... Who cares about what you say. Lancers are very very easier to tank, but a Warrior with tanking glyphs are very good tanks.
The original post asked which class is the most effective. Even by your own admission you say that Lancers are very very easier to tank with, making them the best.
It does not matter if the warrior is or is not a tank, but the fact that he is not the most effective tank.
But being easier does necessary not mean better. Both classes have aggro abilities and thus able to tank. The major difference is that the lancer can stay at one place but up his shield and hold the line, a warrior have to evade, a good one can evade within an area and keep the enemy aggro while the dps kills him of. I believe this is a major reason why warrior has a 5 star difficulty.
I believe that warrior can be as good tank end though it require far more skills from as as player than tanking as a lancer, not making lancer tanking easy just easier.
So play what you prefer and train, good luck :)
Though not a standard tank, a warrior can hold an opponent's aggro. By stinging and pricking the target, whirling between its legs and then backstabbing, the skillful warrior can quite effectively occupy an opponent."
Like a few mentioned above, the warrior is a tank.