Google has not updated PageRank to the public since Dec 2013 and its safe to assume that it will never be updated to the public... PR is not dead its just that in Google's wisdom its no longer being provided to the public... Probably to put a stop to PR related backlink purchases/services etc... Therefore forget PR!!!
Your best option is to use third party metrics but do note they are 3rd party and should only be used as part of a combined assessment... I recommend MOZ DA/PA, Majestic CF/TF...
Finally your focus should be on increasing traffic and ranking positions... Besides PR was only one of many factors and further was never a single metric which explained ranking positions...