Champion's Tunic is the best armour I believe, you get it by finding the first memory. Upgrading it can be a pain without an online guide though. Certain sets have bonuses when each piece is at least level 2 (so you have to find 2 fairy fountains): the climbing set and swimming sets are well worth the upgrades to level 2. Find a combination that you think looks cool and just upgrade those without worrying about the others. I like wearing the Hood with the Champion's Tunic along the Rito Boots. Almost all the armour pieces have identical defense value.