I agree, it's far less important than luma scaling - but I appreciate that madVR does have high quality chroma scaling options available.
Generally I would suggest:
Mitchell-Netravali
Bicubic 75 with the anti-ringing filter enabled
Jinc 3 with the anti-ringing filter enabled
in order of performance impact and quality. (low to high)
Anything above Bilinear is good though, as that can introduce noticeable artifacts, and with the optimized scaling algorithms I'd like to think that few systems would have to compromise and use Bilinear.
Mitchell-Netravali is a good choice if you don't have the performance to enable the anti-ringing filter, as it has very little ringing itself.
You could use Bicubic 75 instead as that's sharper and has the same performance requirements, but it can suffer from ringing.
Bicubic 75+AR gets you 90% of the way there as far as chroma quality is concerned. For the vast majority of sources, this will be indistinguishable from anything more advanced.
And when you have performance to spare, Jinc 3+AR is the best option, though few sources actually require it.
I am looking forward to doing some testing once NNEDI3 is working though.