Best witcher gear is subjective. There's three. Bear school (heavy armor), cat school (attack), griffin school (magic). Most players use bear or cat, since magic is fairly weak overall. You have to find diagrams and crafting components to make the different levels of witcher gear. There's also level requirements for each ranking (normal, enhanced, superior, mastercrafted). All in all, to max out one set, you'll need to be about level 30, need to complete the master armorer and master blacksmith quests and will need at least 6 grand in in-game currency.
Source: Level 36 witcher with mastercrafted bear school set