Never throw it on the floor casually or push it under the sink. First, it’s quite unhygienic and second, you will misplace it. Most of the litter scoopers come with a small attached hook, to hang it on the side of the litter box.
However, there are quite a  few other options. You can always get yourself a nice wall holder, with easy installation. There is a wide choice of customized scooper holders, in all sorts of funny shapes.
You can even get yourself an all-in-one multi-purposes scooper holder, with lots of space to put every piece of cat equipment that you need. Also, like a homemade solution, you can always put your litter scooper in a zipper bag, to prevent any odors or bacterias coming from it.