piermanwalter:

Mario is strongest when he is fully aware of what horrible trials he is getting into. When Mario expects to relax and have a good time, he can’t cope with any disasters. This is why Bowser kidnaps Princess Peach so easily during tea parties and he gets caught by ghosts while vacationing.

Luigi never gets caught off guard because he’s in a constant state of insecurity and paranoia. This can be a drawback in straightforward situations, but this also makes him a perfect ghost hunter.

In times of safety, Wario stresses out over minor details to the point of getting sick, but he half-asses life-threatening situations so hard he can’t even die.

We don’t know how Waluigi behaves, probably because it would be unpublishable.