If it's warm enough outside -- I dump the boxes into the garbage, then take the boxes outside (all 6 of them at varying times), put VINEGAR in them and then fill with water; let them sit for a bit.
If it's wintertime / freezing outside: the bathtub is what I use. I just have to be real careful about the litter getting into the drain!
I just use vinegar & water; it seems to do a great job at cleaning!
Oh and, I also use Lysol wipes to wipe down the outside of the box. 3 of my 6 litter boxes are the hooded kind, so I wipe down the lids with the Lysol wipes, too.
I don't like to use bleach, can't stand the smell & I dunno what bleach would do to my grass out back! So vinegar is it. Cheap, good cleaner.