Answer - The oldest veda is Rigveda.
The Rigveda is the oldest compilation of human wisdom. This Samhita (Collection) is unique. It is not a book, but a compilation composed of several books which can be individually distinguished from each other. The Rigveda represents the earliest sacred book of India. It is the oldest and biggest of all the four Vedas. All the features of Classical Sanskrit poetry can be traced to Rigveda. The present form of this Samhita indicates that the collection is not a single word, but consists of older and later elements.
Various indications of language, style, and ideas prove this point. Different hymns of this Samhita were composed long before they were systematically arranged. Being a compilation of different stages, there is something that stamps the Rigveda with an individuality of its own. It is much more natural in character and form than other Samhitas.
The oldest Veda is
The oldest veda is Rigveda. The Rigveda is the oldest known Vedic Sanskrit text. Its early layers are among the oldest extant texts in any Indo-European language.