1. The simple statements of Buddhist doctrine now found, in identical words, in paragraphs or verses recurring in all the books.
2. Episodes found, in identical words, in two or more of the existing books.
3. The Silas, the Parayana, the Octades, the Patimokkha.
4. The Digha, Majjhima, Anguttara, and Samyutta Nikayas.
5. The Sutta Nipata, the Thera and Theri Gathas, the Udanas, and the Khuddaka Patha.
6. The Sutta Vibhanga, and Khandhkas.
7. The Jatakas and the Dhammapadas.
8. The Niddesa, the Itivuttakas and the Patisambbhida.
9. The Peta and Vimana-Vatthus, the Apadana, the Cariya-Pitaka, and the Buddhavamsa.
10. The Abhidhamma books; the last of which is the Katha-Vatthu, and the earliest probably the Puggala-Pannatti.
Those listed at the top or near the top, such as numbers one to five, are considered the earliest, oldest texts and the most likely to be authentic and the exact words of the Buddha. The later texts and the commentaries and the Visuddhimagga, are held in very high esteem by Classical Theravada, whereas, the Modern Theravada focuses on the earliest teachings of the Buddha.
Bhikkhu Bodhi, Dhammavuddho Thera and others have their doubts, as do modern scholars about the later texts and if they are Buddhavacana (exact words of Buddha) or not. Modern Theravadins probably hold a slight variety of opinions but probably take one of the following:
1. The first four Nikayas in their entirety are Buddhavacana, plus the following books from the Khuddaka Nikaya: Khuddakapatha, Dhammapada, Udana, Itivuttaka, Sutta Nipata, Theragatha, and Therigatha; and the Patimokkha from the Vinaya. (That would still make the Buddhavacana portion of the Tipitaka roughly 30 out of 40 volumes.)
2. All of the above, plus the other books of the Khuddaka Nikaya, plus the other Vinaya books, plus the Abhidhamma, but see them as written by later disciples of the Buddha, who may have been arahants and thus, still worthy to be included in the Canon.
3. All of the Pali Canon as Buddhavacana, but just have some differences in interpretation, where some Suttas are not taken literally because they were not meant to be in some cases (the Buddha specifically states not to take all teachings literally at AN 2.25).