Turtle Armor is incredibly useful as a tank because you're more likely to be targeted when wearing it, so having a lot of health and defense is good especially when your friends can pound the boss.
The only other two things better than Turtle Armor is Solar Flare Armor and Beetle Armor. If you're gonna be a tank, the easiest endgame armor is Turtle Armor, the most difficult is Solar Flare, which is also incredibly good.