I am too lazy to put too much effort into anything for no reason, so my reason for putting too much effort into a lot of panels is 'to practice drawing scenery for my future comic where scenery will be very important'
'For the least reward' though ... it's probably this one:
It doesn't look like much, but I remember this panel being an absolute nightmare for some reason, and I don't even like how it turned out, but eventually I was like 'screw it, I've had enough of this' ... so definitely the most effort for the least reward XD (Maybe it just felt like the most effort :P)
I have more visibly complex-looking panels, but at least I liked how they turned out :'D
I'm the opposite; if I got to a point in my script where I had to draw a particularly tedious panel, it would have to be a 100% necessary panel for my story that I'm absolutely GUARANTEED to use, otherwise it would never even make it past the sketch before I decided to cut it, even if I had to change the scene XD