Daughter! I love hiking with my wife, my son, my mom, my mother-in-law, and a wide variety of friends and acquaintances -- as well as going alone -- but watching my daughter go from baby-in-backpack to a comparatively equal and always-enthusiastic four-season, all-terrain partner-in-crime has been one of the great joys of my life.
Mine is my dog Marlie. For those of us that have four legged friends for long term hiking partners the answer is clear. Unconditional loyalty and never a complaint.