There is no concept of best in the TeX world.
As mentioned in the comments, most active key-value handling packages are
pgfkeys and the
l3keys. The latter is also inspired by
pgfkeys however it requires a minimum level of LaTeX3 knowledge which might be a bit baffling for newcomers since it doesn't have a comprehensive introduction and manual (yet!).
pgfkeys on the other hand is pretty straightforward and has a very very comprehensive description placed within the graphics package TikZ/PGF manual (hence the name
pgfkeys which TikZ uses extensively).
Both packages are pretty matured and can handle pretty complicated tasks.
Personally I would suggest starting with
pgfkeys if compared the two. However, if a minimal L3 knowledge is present they are more or less equivalent in terms learning curve.
Even I use
pgfkeys mostly and extensively, my personal favorite is
yax which is from one of my favorite manual writers Paul Isambert. Yax gives a more Pythonic dictionary-based attribute parameter syntax which is not very common in TeX universe but once you get a hold of it, it really pays off. Though it might not be the most general or the most up-to-date it still gets the job done.