Best is subjective. However, PowerPro is very popular. That is the one I use. What type of application, rod/reel set up, etc. are you looking to start with? Typically, 10#-30# braid on spinning gear. 30#-80# for bait casters. Below is a comparison of diameters.
I would suggest 20# is a good starting size for spinning and 40-50# for bait casting.