For me it's easily refining liquid morkite. It takes forever. I swear half the team is off staring at a wall while you get the pipes built. Lastly, I can't quite out my finger on it, but the refining portion feels so scripted and boring....Every other mission type is a blast.
arquarqs by far. really boring mission and because I'm a gunner at heart if the right team layout is not available I can waste resupplies easily depending on map layout and area. I find vertical maps to be the most challenging think Crystalline Caverns