It is very hard to actually determine what the best mustahabb act is and we will explain why that is so:

Different actions have been considered the best mustahabb act in different hadiths.

Awaiting the help and aid of Allah (swt)[1], prayer[2], sending salawat on the prophet (pbuh) and his progeny (as)[3], quenching the thirst of the thirsty[4], the love of Imam Ali (as)[5], making others happy[6], jihad[7], continuously doing a good deed[8], visiting the grave of Imam Husain (as)[9], night prayer (Salatul-Leil)[10], loving for Allah (swt) and hating for Allah (swt)[11], prayer on time and in the beginning of its time[12], doing good to your parents[13], feeding others[14], doing whatever is harder[15], etc. Aside from these, there are hadiths that speak of some acts being the best because of a certain time or place factoring into them, such as doing something on Jum’ah (Friday)[16], in the holy month of Ramadhan[17], on the night of the fifteenth of Sha’ban[18].

So what is the best mustahabb act one can do?

As you can see, there are many hadiths that speak of the best mustahabb act one can do; how can one draw a conclusion from all of them and find an explanation for why they have spoken of more than one “best act”?

1- Difference in the reasons behind them and the benefits they entail:

Each and every one of these acts that has been considered “the best”, has been considered so because of a certain reason and perspective; each of them has its own criteria for being “the best”. For instance, certain reasons make the night prayer the best act, while others call for jihad to be the best. Take jihad for example, when the time for it comes, it is clearly the best act one can do and all other acts are inferior to it. This is one way to explain how all of these acts can be “the best” at one time. It is because of not being able to solve the problem in this way that led to a portion of the adhan being omitted during the second khalifah’s time. They weren’t able to explain how prayer was “the best of all acts” and at the same time, jihad was also the best act of all, so they just ended up omitting the “حی علی خیر العمل” part of the adhan altogether![19]

Therefore, a difference in reasons, and criteria can be a good way of explaining the true meaning of these hadiths. Some of them have very significant social benefits, such as jihad. Some of them bring along personal benefit, while a third group aren't even actions and have to do with our beliefs, such as loving Imam Ali (as). Some hadiths haven’t even mentioned a specific action and have generally said the hardest act is the best of all acts.[20]

2- Difference of circumstances and situations:

Sometimes it is the situation the person is in that explains the variety seen in these hadiths. It was said above that the reasons behind different actions might make each of them “the best” act, so what mattered there was the action itself and the benefits it entailed, while here the thing that makes the difference doesn’t have to do with the act itself, it has to do with the circumstances the person is in, what time and place he is in and so on. This is also another way of explaining how these actions can be “the best” all at once. For instance, during the advent of Islam when the Muslims faced a rough time, or during the times when the Shia were under a lot of pressure, awaiting the help of Allah (swt) and not losing hope were the best of all acts and worship; the reason being that if the Muslims were to lose hope, everything would be jeopardized and would face elimination. The same goes with other actions that were mentioned above. Clearly, in normal conditions, jihad isn't the best act for women. The night prayer isn't the best act for a person who has just embraced Islam and still hasn’t learned how to perform his daily prayers.

Conclusion:

It can be deduced from all of these hadiths that there is no one best act in Islam. It can't be said that under all circumstances and in all times such and such act is the best. As was mentioned, some of the acts that have been considered the best have different reasons behind them, while others become the best act as a result of certain circumstances. Nevertheless, those acts that have been mentioned in our hadiths are of the best acts out there.