I like various people’s outfits: Machiavelli’s Brotherhood garb, Sofia Sartor’s dress in Revelations, Maria Thorpe’s non-Crusader outfit, Leonardo da Vinci’s outfit in both games he’s featured & some of the fellow Assassins’ robes in the Brotherhood trailer & in Revelations.
As far as Assassin’s robes/armour though, my favorite is split between:
The Armor of Altair [which reminds me…I wonder why we never saw Altair wear this armor during any of his memories in Revelations…]

&
The Helmschmied Drachen Armor