No one best. Two most popular ones are fixodent and poligrip. Different patients prefer each one. Consider a dental check-up (even if you have full dentures) to see if they need to be relined or remade so that you don't have to use any adhesive (or less). Also consider implant retained prostheses.
Many. Much depends upon the fit of your dentures. Normally patients like the cream adhesive better than the powder or the pads. Fixodent is a very popular one. If adhesive just doesn't work, consider a reline or possibly new dentures.
Denture Adhesives. The choice of a denture adhesive is a personal preference. There are 3 categories to choose from, pastes, powders and pads. I suggest comparing all 3 and choosing the product that work best for you situation. Your looking for the product that improves denture retention and stability the best.