Binoculars

You carry these to see a fight before it starts. Lie on the ridge above a base you plan to raid and count the lit windows, the turrets, the guy farming on the roof. Or check Launch Site from the hills before you run in with a pocket full of scrap. No other tool this cheap gives you that view.

They cost 40 metal fragments and no bench, and the research is cheap, so there is little reason not to keep a pair in the base. Even the mortar says in its own description that it pairs well with binoculars: one player spots, the other corrects the shots.

The scopes win when you also want to shoot what you see. For pure watching the binoculars are cheaper, and losing them when you die costs you almost nothing.