Snow White. And i've thought so ever since i was about four years old. Her dress has a very fitting combination of colors, with three of the four primary colors present (green being the only one absent), and it is beautifully crafted. Her puffy sleeves looks like some kind of candy, and her big collar and cape (yes, she does wear a short cape in a few scenes) are the icing on the cake.
However, he jumped the shark when he got new civilian clothes from episode 37 to the last episode of the series. Note then he wasn't the only one to change clothes, Venusia (Hikaru Makiba) also changed clothes.
Another guy best dressed cartoon character is Franco-Belgian comic character Lucky Luke
In the case of Tintin, some liked the jeans he weared in "Tintin & the Picaros" (who replaced his plush-fours then we know in French as "golf pants") while other disliked it.
Just for the record, Darkwing Duck would probably have been my #1 choice (if not at least tied for #1), but his lack of....well, you know....pretty much cost him a point or two.
Spirou and Fantasio from the main Dupuis comic series of the same name. Both characters clothes varied depending the years and the artists' tastes.
Boule's mother in Boule and Bill have also beautiful dress, especially in the end-1960's and 1970's shots. It's almost never the same in each gags pages she appear.
I agree with Stephane Dumas, Lucky Luke's dress is what i can consider as a true man.
The rare times i see Ren and Stimpy dressed are still very elegant, even in the Games episodes.