The larger the number is, the more hydrophobic the amino acid. The most hydrophobic amino acids are isoleucine (4.5) and valine (4.2). The most hydrophilic ones are arginine (-4.5) and lysine (-3.9). This is very important in protein structure; hydrophobic amino acids tend to be internal (with regard to the protein's 3 dimensional shape) while hydrophilic amino acids are more commonly found towards the protein surface.
1. Kyte J, Doolittle RF (May 1982). "A simple method for displaying the hydropathic character of a protein". J. Mol. Biol. 157 (1): 105–32. PMID 7108955.