On professional, it depends on the boss and spirts. The Stasis Stream (blue) is a life saver when trying not to cause too much damage but quickly deal with spirits. All the Ghosts that you can not capture in the trap, I would freeze and blow up. But some of the spirits, like the black slime ones can only be defeated using just the slime blower. The final boss, I felt the Meson Collider (orange) was the best. It was powerful to deplete his energy and spot on accuracy to destroy the energy towers whenever he recharged. All in all, it depends on the situation; all 4 weapons are good at some point.

On casual, yeah the best weapon is the blast stream because ghosts and spirits health is low and they're easier to defeat. Don't really need the other weapons.