edit: I realize I sort of jumped into the deep end here. If you don't know what "allele frequency" means, see here and let me know if you have follow up questions.

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Answering the question of what allele frequencies are currently changing the fastest is not necessarily that easy, as human generation times are long enough that it's not that easy to observe it happening in real time.

There have been a number of statistical techniques developed to identify very recent positive selection. Summarizing the literature in a reddit post would be difficult, so I'll try to give just a few highlights. One of the most exciting developments in this field is the recent arrival of so called ancient DNA, or the ability to sequence/genotype individuals who have been dead for thousands of years. There is a paper currently posted on a preprint server which examines the genomes of 83 humans who lived in Europe in the range of 4000-8000 years ago, and compares their genomes to a number of present day populations within Europe. They find just 6 loci that show robust evidence of recent positive selection (i.e. very rapid allele frequency change). Two of them, at genes called SLC24A5 and SLC45A2 are associated with (but not entirely responsible for) differences in skin pigmentation between Africans and European.

Another one, called rs4988235, is largely responsible for the ability of Europeans to continue digesting lactose (i.e. drinking milk) well into adulthood, and has been known of for a while.

Another example is a marker called rs12913832, located near two genes called OCA2 and HERC2, which is in large part responsible for blue eyes and possibly also associated with lighter hair pigmentation (1,2,3)

The other two strong signals found by the ancient DNA studied cited above are in two genes related fatty acid metabolism and circulating vitamin D levels, suggesting possibly adaptation to diet.

These are just regions that have been identified as the strongest signals within the continent of Europe. It's pretty widely recognized at this point that to the extent that positive selection has had any impact on recent human evolution (and it's not all that clear that the effects have been that major), the effects differ from one region to the next.

I won't give a whole rundown, as that would make this even more ridiculously long (and because Europe is, predictably, the best studied region for this question), but for example populations of Tibetan highlanders appear to have undergone very recent adaptation in a region of their genome which allows them to better tolerate the thin atmospheres they live in, and the it appears that the allele that they are using to do so actually came from ancient interbreeding with an archaic group of now extinct humans called Denisovans, who are more closely related to Neanderthals that they are to us.

In Africa, for example, there has also (in some regions but not in others) apparently been strong selection for a number of alleles which allow their carriers to digest milk into adulthood, but the alleles that have been targeted by selection in Africa are different than the ones that have been targeted in Europe.

edited to add: It should be emphasized, however, that it maybe the case that none of these regions are any longer experiencing strong natural selection, and therefore may not be changing very fast in frequency "right now". I would expect, however, that we will see studies within the next few years that sequence/genotype large numbers of individuals spanning multiple generations of presently living individuals and try to identify regions which are currently the target of natural selection. Whether or not we will find anything interesting doing this remains to be seen.