Medical Exoskeletons are probably the most useful. Each Medical Exoskeleton X provides a 23% cooldown reduction for all of your abilities, allowing you to buff yourself more often and keep crowds under control. They also provide 4.5 health regen/second and 80% toxin resistance. Combined with end-game shields and good use of cover, you should never come close to dying. Two of these on your armor is godlike.
Kinetic Exoskeletons are also good if you're more of a frontline type of character. Each Kinetic Exoskeleton X provides 35% shield recovery, 50 shields, and 26% mobile accuracy. This is good for tanks as they can recover their shields more quickly and stay in the heat of battle longer. The mobile accuracy helps you with dealing out damage while staying on the move to avoid being hit.
The benefit of having your tech/biotic/combat abilities available to you more often outweighs the advantages of higher shield recovery and better mobile accuracy, though. Try equipping one of each or two of either and see which setup suits your play style better.