Which are considered to be the oldest suttas?
I was told that the Sutta Nipata is a one of the oldest collections. I'm no Buddhist scholar so I can't help with any great textual analysis of the sutta. However Thanissaro Bhikkhu says here that the The Parayana Vagga is quoted in other (more recent) parts of the Pali Canon so this makes it a good candidate to be considered one of the oldest. To quote the relevant part of the passage


  There is evidence that these sixteen dialogues were highly regarded
  right from the very early centuries of the Buddhist tradition. [...]
  Most of the Cula Niddesa, a late addition to the Pali canon, is
  devoted to explaining them in detail. Five discourses — one in the
  Samyutta Nikaya, four in the Anguttara — discuss specific verses in
  the set