Do you mean objectively or personally?
Either way, it would probably be these three, read in this order.
1. Dhammacakkappavattana Sutta (SN 56.11)
2. Mahasatipatthana Sutta (DN 22) (although I personally prefer MN 10 - less 'insertions')
3. Kaccayanagotta Sutta (SN 12.15)
Kalama Sutta would have to be there or there abouts too.
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"The uprooting of identity is seen by the noble ones as pleasurable; but this contradicts what the whole world sees." (Snp 3.12)
"To argue with a person who has renounced the use of reason is like administering medicine to the dead" - Thomas Paine