Great question, it actually falls inline with my theory that there are different tiers of titans that serve different purposes.
It seems like titans were genetically designed weapons of war and each type of titan fills and serves a different role. I like to put them into 3 categories.
Drone/Grunt Titans: The have the appearance of a normal human being just very large although some are abnormally deformed. They are the mindless titans that make up the bulk of the army, pretty much programmed to do one thing, kill. They are able to follow directives when ordered by a "coordinate" as shown by Eren and Big Foot
Agent/Warrior Titans: They have no skin, are lean and muscular and range from 14m-60m. They are shifters who's roles range from infiltration, information gathering or breaching enemy defense. Their titan forms have special offensive and defensive abilities which can be used to fight other titans or breach walls, avoid capture etc.., they are also able to shift in and out of form easier then other titans in order to escape when needed (which could be why they have no skin)
Leader/Commander: Titans that look more like monsters with a feral looking appearance, sharp teeth, elf like ears, possibly fur? Most likely strongest of all titans as Eren despite his inexperience seemed much stronger then Annie (Annie was only able to fight Eren due to superior fighting skill and speed) and overpowered and nearly killed Reiner Able to issue commands to drone titans and possibly intimidate warrior titans into doing their bidding.