I don't know what is "best", and I can't remember how or why I setup this way, but what I use is:
Bit Depth - 16 bit
Dither - MBIT +
Noise Shaping - None
Dither Amount - Normal
Seems to work pretty well for me.
I use the following sequence:
For Bit Depth I use the Izotope Bit Depth set to - 16 bit, Dither - MBIT +, Noise Shaping - ultra, Dither Amount - Normal, and I also check the silence harmonics box.
For Respampling I use the Izotope SRC plugin set to the simplified setting with the slider moved all the way to highest quality. and as Bean stated I always resmple before doing the bit depth processing. It is always better to do all you processing in 24bit before changing to 16bit. Although I doubt any of use would know the difference without test equipment to show it.
Peace OOK