Most stable amino acid at physiological pH is
Histidine
Lysine
Arginine
Glycine
Histidine plays unique roles in enzymatic catalysis. The pKa of its imidazole proton permits histidine to function at neutral pH as either a base or an acid catalyst  without  the  need  for  any  environmentally  induced shift. 
Harper's Illustrated Biochemistry 30 th edition page no 22