– Of these dhammas, Upāli, of which you may know: 'These dhammas are not utterly conducive to nibbida, to virāga, to nirodha, to upasama, to abhiñña, to sambodhi, to Nibbāna, Upāli, you may definitely hold: 'This is not the Dhamma, this is not the Vinaya, this is not the instruction of the Teacher'.
Aṅguttara Nikāya: I'd recommend looking at Bhikkhu Bodhi's Thematic Guide and reading the discourses by whatever topic interests you. A discourse that usually stands out in this nikāya is AN 3.65. (I also like 3.61, 3.68 & 3.69)
Khuddaka Nikāya: I recommend reading fully the Dhammapada, Udāna, Itivuttaka, Sutta Nipāta, Theragāthā & Therīgāthā. They are small and easy to read. If I had to choose one, I'd say the Sutta Nipāta.