For me, it was Light Armor, which is odd since I originally wanted to make a mage.
wow you must get hit alot. onehanded was my first. i still dont have light armour at 100 and thats all ive ever used, its probably my own fault for putting a sheild in the way, then its smithing block, after that my highest are sneak speech and lightarmour. but the last three are like all 80s.
strangly my main character has over 150 hour in him yet so few lvl 100 skills, my lvl 25 mage is 100 in illusion and on the way in sneak and speech.