This section of quotes will most likely be one of the smallest since Koenma isn't in all of the Yu Yu Hakusho volumes like the others. So here is what I've got for now:

"Ha Ha Ha... Her hair. Surprised?"-Vol. 2

"In other words, he's an idiot!"-Vol. 2

"It's pointless to try to judge his character while he's a bodiless ghost."-Vol. 2

"We miss tomorrow and you've got a half-century wait on your hands."-Vol. 2