With the right disguise, Agent 47 (Hitman) can blend into any environment almost perfectly.
Camille Leon (Kim Possible)
Robbie Rotten (Lazy Town) claims himself to be the master of disguise, having a costume for every possible scenario
Chameleon/Dmitri Smerdyakov (Marvel Comics) has not only masks and costumes, but has gadgets that allows him to mimic superpowers (if he impersonates a superhuman).
Masters of disguise, Agents of Hydra (Marvel Comics) can blend in almost anywhere as a group of assassins disguised themselves as police officers when plotting an attack on Nick Fury.
Mystique (Marvel Comics) uses her shapeshifting abilities to the fullest, and being a trained actress, she can perfectly disguise herself as needed.
Decoy Octopus (Metal Gear Solid), master of disguise.
Queen Chrysalis (My Little Pony: Friendship is Magic)
Obito Uchiha (Naruto) proved himself to be a consummate actor, fooling everyone he is the happy-go-lucky idiot Tobi, then the globally-feared Madara Uchiha.
Kurozumi Kanjuro (One Piece) was so skilled in acting that he actually lives the role he plays, risking his own life to completely fool the Kozuki Family.
Duplica (Pokemon) is a girl that can disguise herself as anybody in matter of seconds.
Copycat, (Pokemon) like her name implies, copies people's looks just for the fun of it!
Sedusa (The Powerpuff Girls)
VFD members like Count Olaf (A Series of Unfortunate Events) are highly trained to become masters of disguise.
Through the usage of synthetic skin, Peyton Westlake (Darkman) can impersonate others to a high degree of success.
Sly Cooper (Sly Cooper), is in the art of disguising and fooling others.
Through the act of innocence, Dain (Deltora Quest) was capable of convincing everyone around him, even the audience, that he was the rightful heir of King Adin, destined to wield the Belt of Deltora to defeat the Shadow Lord and save the kingdom from his tyranny...
...when in fact, he was a sadistic spy created by the Shadow Lord whose goal was to undermine the heroes' efforts once and for all, and no-one ever suspected this until he revealed it himself.
Al the Actor (Top Cat)
Mimic (IDW's Sonic the Hedgehog), disguised as Sonic.
Walter Naugus (Archie's Sonic the Hedgehog)
Harvey Who (Archie's Sonic the Hedgehog)
Max Goof (Goof Troop) dresses as Powerline and has sunglasses on so nobody recognizes him.