Your question is just a little bit too open ended. "Best" with cost being no object....probably Griggs. "Best" handling upgrade over stock without mortgaging the house.....Canadian Mustang has a nice kit (I have it on my 65 fb along with a Shelby mod), it'll vastly improve your stock setup. Then there is TCP, Cobra Automotive and Global West in the middle and plenty of other vendors with stock or stock upgrade kits. Redefine your question and we'll try to help more.....
If you're on a low budget like me, let me tell you what I've done and take it from there. My stock springs are in very good shape. I left them alone, put KYB high pressure shocks, 1" front sway bar and an export brace. I would have put a straight Monte' Carlo bar on too, but the one I got from CJ Pony parts is a peice of fesis.
I also want to lower the upper arms and lower the car 1". But as it stands right now, what I've done has made a huge difference! The car rides a lot firmer, I really don't see a need for heavier springs. It handles very neutral, I'm very pleased.
Maybe for more agressive driving you'd want more, but this a great starting point!
You can also go with TCP stuff, but they've raised their prices so much lately that it doesn't seem worth it anymore. Mustang Plus has a kit that incorporates some nice TCP components with other cheaper pieces, but its almost as much as one of the nice Global West kits. Personally I'm probably going to go with a Global West kit if I don't get the Rod & Custom Rack and Pinion setup in the front.