What is the best thing in life, according to the Buddha, excluding nirvana?

Everyone has great suggestions, so far. I'd like to present a wholesome alternative, found in the Upaḍḍha Sutta (SN 45:2).

...Admirable friendship, admirable companionship, admirable camaraderie is actually the whole of the holy life. When a monk has admirable people as friends, companions, & comrades, he can be expected to develop & pursue the noble eightfold path.

The whole text can be found here, as translated by Thanissaro Bhikkhu.

Not to associate with fools, maintaining relation with the wise, sacrificing toward what's worthy to sacrifices, is the root of all best things, the foundation of a blessed life here, the next, and bliss beyond, good householder. Feel blessed?

Apart from Nirvana, Dhp 178 tells us that stream entry is an achievement superior to anything else in the world or the heavens.

Far better than sovereignty over the earth, or far better than going to the abodes of the devas, or far better than ruling supreme over the entire universe, is (the attainment of) Sotapatti Fruition.
Dhp 178

But I guess in this question, you're looking for other more worldly things.

There are many blessings in life indeed, according to the Mangala Sutta.

The Mangala Sutta:

1. "Many deities and men longing for happiness have pondered on (the question of) blessings. Pray tell me what the highest blessings are.

2. "Not to associate with the foolish, but to associate with the wise, and to honor those worthy of honor — this is the highest blessing.

3. "To reside in a suitable locality, to have performed meritorious actions in the past, and to set oneself in the right direction — this is the highest blessing.

4. "Vast learning, skill in handicrafts, well grounded in discipline, and pleasant speech — this is the highest blessing.

5. "To support one's father and mother; to cherish one's wife and children, and to be engaged in peaceful occupations — this is the highest blessing.

6. "Liberality, righteous conduct, rendering assistance to relatives, and performance of blameless deeds — this is the highest blessing.

7. "To cease and abstain from evil, to abstain from intoxicating drinks, and diligent in performing righteous acts — this is the highest blessing.

8. "Reverence, humility, contentment, gratitude, and the timely hearing of the Dhamma, the teaching of the Buddha — this is the highest blessing.

9. "Patience, obedience, meeting the Samanas (holy men), and timely discussions on the Dhamma — this is the highest blessing.

10. "Self-control, chastity, comprehension of the Noble Truths, and the realization of Nibbana — this is the highest blessing.

11. "The mind that is not touched by the vicissitudes of life, the mind that is free from sorrow, stainless, and secure — this is the highest blessing.

12. "Those who have fulfilled the conditions (for such blessings) are victorious everywhere, and attain happiness everywhere — To them these are the highest blessings.

Conventionally speaking, the samādhis of the Āryas are "not suffering." I would like to cite a few texts here, one of them is a Pāli sutta, another is a commentary on the Prajñāpāramitā, and the last is an ancient Dhyānasūtra of unknown sectarian affliation.

That being said, all these citations being present, these (referring to the samādhis of the Āryas) are only "not suffering" conventionally speaking. All of the concentrations of the Āryas, unless they have Nirvāṇa, are "suffering" ultimately. It is only "compared to the saṃsāra of the worldling" that they are "not suffering." Evidence of this comes from the Buddha himself in the EBTs and beyond, then he recounts that he mastered such-and-such dhyāna or such-and-such formless samāpatti and found it to be still impermanent, still impure, still suffering. That is why he did not achieve Bodhi under the tutelage of his foremost gurus, Rudrakarāmaputra and Āḷārakālāma. He found their samādhis to come short of what he was looking for. Because of his profound faculties, he was not led astray by the sweetness of the samādhis, and he did not take subsequent rebirth via the fruits of said samādhis (i.e. "as a formless deva" or "as a deva with form").

From the Santatarasutta at Iti 73 as translated by John D. Ireland:

“Bhikkhus, the formless is more peaceful than the form realm, and cessation is more peaceful than the formless.”

This refers to 1) the peacefulness of existences in the heavens of form, 2) the peacefulness of existences in the heavens that are formless, and 3) the peacefulness of the cessation of existence ("bhavanirodha"). Furthermore, it also refers to 1) the peacefulness of the yogic equipoise of the dhyānas, 2) the peacefulness of the yogic equipoise of the samāpattis, and 3) the peacefulness of the yogic equipoise of Nirvāṇa. Whether we take it to refer to one of these three or both of these three, the general principle holds. The formless samāpatti is more peaceful than the rūpadhyāna, and Nirvāṇa is more peaceful than either.

From the Mahāprajñāpāramitopadeśa as translated by Venerable Migme Chödrön (she translates "samādhis" as "concentrations"):

[...] the yogin pursues his reflections and asks himself why beings are attached to this body. It is because of pleasant feeling. How? From the meeting between the six internal organs and the six outer objects the six kinds of consciousnesses arise. From these six consciousnesses arise the three kinds of feelings, unpleasant feeling, pleasant feeling, neither unpleasant nor pleasant feeling. Pleasant feelings are loved by all beings; unpleasant feelings are hated by all beings; as for the neither unpleasant nor pleasant feelings, people neither reject them nor cling to them. Thus it is said: Evil-doers and monks, Gods, humans and small worms: Amongst these beings divided among the five destinies in the ten directions, There is not one that does not love happiness and hate suffering. Out of error, mistake and ignorance, They do not know nirvāṇa, the abode of eternal bliss. Considering pleasant feeling, the yogin truly knows that it contains no happiness but only suffering. Why? Happiness, i.e., ‘true happiness’ is free of errors. And yet all the pleasant feelings of the world come from mistakes and contain no reality. Furthermore, while greedily seeking the happiness of pleasant feeling, one will encounter great suffering. Thus it is said:

Those who go to sea encounter heavy winds. The waves rise up as high as the Kālaparvata. Those who go into the army to fight across very dangerous paths and perilous gorges. These many great sufferings all come from attachment to happiness and to cupidity."

This is why we know that pleasant feeling can give rise to all sorts of suffering. Furthermore, although the Buddha spoke of the three kinds of suffering, one of them, that of pleasant feeling, merits the name of suffering because in it happiness is rare. It is like a bushel of honey which, when thrown into a big river, loses its smell and its taste.

Question. – Happiness such as it is conceived in the world, having error as cause and condition, is suffering. But the concentrations practiced by the saints give rise to a pure happiness which itself is real happiness. Why? Because this happiness is not derived from delusion or mistake. How then could it be suffering?

Answer. – It is not suffering. [...]

The bolded section identifies the relevant point. The preceding material establishes how, according to the treatise, even "happiness" is suffering.

From a Dhyānasūtra quoted in the Mahāprajñāpāramitopadeśa:

Avoiding desires and bad dharmas, a person enters into the first dhyāna furnished with examination and judgment, coming from detachment, which is joy and happiness. Avoiding the flames of lust, he is endowed with clear cool absorption. Happy like a person who, tormented by the heat, enters into a cold pool. As in the case of the poor man who has found a treasure, vitarka of a great joyfulness moves his mind. He analyzes it: this is vicāra. This is how he enters the first dhyāna. He knows that vitarka and vicāra disturb his mind. Although good, he must separate himself from them, for it is only on a calm sea that the movement of the waves is not seen. When a very weary man lies down to sleep in peace, any call to him strongly disturbs his mind. In the same way, for the absorbed man in dhyāna, vitarka and vicāra are a torment. That is why, avoiding vitarka and vicāra, he succeeds in entering the sphere of unified consciousness. As a result of his inner purity, he finds joy and happiness in absorption. Penetrating into the second dhyāna, his joy is lively and his mind is very happy. An absorption where concentration is very strong is calm and free of smṛti. Annoyed by prīti, the ascetic wants to get rid of it in the same way that he has already eliminated vitarka and vicāra. It is because of feeling that there is joy. If joy is lost, sadness is experienced. Renouncing pleasant bodily feeling, The ascetic abandons memory and methods. The saint is able to reach this renunciation; for other people, this renunciation is difficult. When one knows the torments of happiness, one sees the grand immobile peace. When daurmanasya and prīti are eliminated, duḥkha and sukha still remain to be cut. Purified by equanimity and reflection, the mind penetrates into the fourth dhyāna. The sukha present in the third dhyāna, transitory and changing, is suffering. In kāmadhātu, the ascetic has cut the daurmanasya; in the second dhyāna he has eliminated the prīti. This is why the Buddha Bhagavat said, in the fourth dhyāna, having cut the daurmanasya and the prīti, it is necessary now to cut duḥkha and sukha.

(Dhyānasūtra T216 as translated by Venerable Migme Chödrön)

The samādhis of the Āryas are happiness, but they are not Nirvāṇa.

It would only be appropriate to conclude with the gāthā that was omitted in the initial quoting of the Santatarasutta in this post:

Those beings who reach the form realm and those established in the formless, if they do not know cessation, come back to renewal of being. Those who fully understand forms without getting stuck in the formless are released into cessation and leave death far behind them. Having touched with his own person the deathless element free from clinging, having realized the relinquishment of clinging, his taints all gone, the Fully-Enlightened One proclaims the sorrowless state that is void of stain.

(same as first quotation, translation from John D. Ireland)