Simply the best...

I think "ballerines" (also called ballet slippers) are simply the best. This little piece of material can be worn days and nights if you dress them up. You can wear them with jeans, shorts, skirts, dresses...
My favorites are french from Repetto. They are around 150 Euros but definitely worth it. http://www.repetto.fr/boutique/chaussures/femme/ballerines.html?SID=20bd51fcb21f3fab0be31275da0ed533