Sorry if this is bit off topic, but what's the best flat dithering and resampling algorithm at the moment?

While often used together, at least when downsampling bit depth, those are quite independent of each other.

Dithering and noise shaping (very frequently used with dithering) are all over the map. Many people have beliefs in some particular approach(s), most people probably can't hear any difference between any of them under test conditions, except perhaps the most extreme variations on the "bad" side.

Resampling is also open to many approaches. Part of it is simply calculation accuracy, part of it is what flavor, extent, and accuracy of filtering is used with the resampling. Some programs are definitely better than others; some hardware resampling is rather crude and poor.

Audition has very good resampling and quite a selection of dithering and noise shaping options. There are a couple of freeware resampling applications floating around that you could probably locate with the search. There are also quite a few for sale; Ozone's often gets good mention.