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.
keyreaderand I use another option as well. Horses for courses - asking which horse you'd suggest entering into an arbitrary race isn't a useful question. Which race? A minimal example document which shows what you are trying to do would greatly improve your question and the quality of answers. – cfr Apr 23 '16 at 20:48expl3and itsl3keysseems also plausible; and it's going to be the “main future” of LaTeX. In a general case, I would personally choose that, andpgfkeysin second place because I like the easiness and flexibility to define new keys. – Manuel Apr 23 '16 at 21:01keyvalandxkeyvala nightmare to use in comparison with bothpgfkeysand, expecially,l3keys. I never did get to grips with them and am greatly relieved thatl3keysandpgfkeyshave come along so I may never have to. I findl3keysby far the easiest for complex stuff andpgfkeysby far the easiest for simple stuff. But I would mainly usepgfkeysorl3keysbased on what I was doing - if it is TikZ or something similar,l3keysbecomes at least a bad dream, andpgfkeysmakes much more sense. But my kilometres have evidently differed from yours ;). – cfr Apr 24 '16 at 00:21