Algorithms for down sampling an image? Bicubic sampling is pretty good for up sampling an image and making it larger,  but is it a good choice for down sampling as well? Are there better choices?
When Sean and I wrote stb_image_resize we chose Mitchell for downsizing. Mitchell is similar to Cubic, you can read about the cubic class of sampling filters in Mitchell Netravali 1988. They are all pretty similar and will get you very similar results.

I can't find any written record between Sean and I of why we decided to go with Mitchell, but if memory serves we just resampled a bunch of images and used the algorithm that we thought looked best. I wouldn't say that there is one authoritative or best filter, you should use the one that looks best on your data.

Edit: Like joojaa says, a windowed sinc filter is also good, if not quite as cheap. You can find some implementations here.