What is the oldest form of Western Liturgy that we have?

[Leevo]

I know the oldest known form of liturgy of the Eastern churches is the Divine Liturgy of St. James. What is the oldest form of the Mass that we have? Or is the Divine Liturgy of St. James celebrated in the West as well?

 

[Rhamiel]

probably the unreformed Ambrosian rite?

after Vatican II, even the old liturgies were reformed so I am not sure?

that was from 350 or 370
so the Church had already been in Rome for about 300 years at that point
so that can not be the OLDEST

 

[Virgil the Roman]

The Mass of St Gregory the Great, essentially the Tridentine rite prior to the medieaeval era. It was shortened from the mediaeval prayers added, such as the Judica Me and the Last Gospel; but in its essence, it is the substantially the same. The Roman Canon is most ancient anaphora or Eucharistic canon/prayer, the West possesses.