PGR depends on the PGR - ie must of us would say stay away from Paclobutrazol. Others aren't aren't as scarey. I like Nitrozyme and DynaGro Foliar (or Grow) for foliar sprays, but it also depends on why you're spraying. If I have a CalMg deficiecy, then I spray Calmg. If I'm dealing with stress issues, Superthrive would be in order. If I'm trying to get a smaller plant to catch up to a bigger plant pre-flip, I like DynaGro, BioWeed, Nitrozyme or Verde...
Then there is all the foliar options that deal with insects and fungus outbreaks/prevention.
I certainly don't think there is a best one, but there are a lot of good ones.