IMHO, there is no "one best article spinner/rewriter", simply because what works for someone else, or what someone thinks is an 'awesome addition', could just be the total opposite for someone else!
Case in point, I hear a lot of good things about the free version of SpinnerChief...for me though, I'm not a big fan of any app that takes more then 3 clicks to open and when it does get around to it, doesn't remember the settings I set the last time I used it...so that app sits on my HD, rarely used and quite honestly...now that I'm thinking about it, I should just go remove it completely!
I'm a firm believer that to get the most out of ANY spinner, you need to spin more then just synonyms, but full sentences and/or paragraphs...thus I use some old app called Spin Master Pro (IIRC) that I picked up as a freebie from someone on another forum and have been using it often!
I guess the best way to find what works best for you is to try and find a few trial version and give them a run. At worst, watch a few training vids and see how well you like the GUI...if you think the apps does what you're looking for it to do, in a way that you'll find it fairly easy to use, then by all means, fork out the dough and buy it!