If I understand it correctly, Nirvana is the liberation from the endless cycle of rebirth, thereby terminating all sufferings.
Is Nirvana a goal which every Buddhist must achieve?
Is there any other things more important than attaining Nirvana?
No, Nirvana is not the goal of all Buddhists. According to Sakya and Nyingma schools of Tibetan Buddhism, Buddhist liberation methodologies can be divided in three major classes, corresponding to sentient being's mental capacity:
The three capacities model should not be confused with the three yanas model (Hinayana/Mahayana/Vajrayana) which can be seen as three levels spanning the second and third capacities, nor with the three schools model (Theravada/Mahayana/Tibetan Buddhism) which are three alternative paths, with beings of all three capacities found in each school.
Another model defines goals according to one of the Six Worlds the being finds itself in:
There is also a nine-yanas model that provides a finer classification of enlightenment contexts, with a slightly different provisional goal utilized in each. Please note that all models exist solely for their explanatory value and are vastly simplifying reality, which is a lot less pigeonholed.
To answer your second question, is there any other thing more important than attaining Nirvana, this is like a school student asking, is there any other thing more important than finishing school. For the school student to finish the school is the most important thing. Nothing else can happen until then.
In the grand scheme of things, ending suffering and with it the endless cycle of rebirth is the ultimate goal of all forms of Buddhism.
However, while Theravada Buddhism focusses on personal enlightenment, other forms like Mahayana focus on the Bodhisattva ideal: Postponing enlightenment until all other lifeforms are free from suffering. In the Theravada tradition this Bodhisattva is postponing enlightenment until one can become a Buddha.
So to answer your questions:
Since none of the 5 answers so far (at the moment of this writing) provide any external references that directly tackle this question, here are some from the actual suttas and Buddha's own words as preserved in the pali canon.
In MN 73, the wanderer Vacchagotta said this to the Buddha after hearing his Dhamma exposition and before going to him for refuge:
Just as the river Ganges inclines towards the sea, slopes towards the sea, flows towards te sea, and reaches the sea, so too Master Gotama's assembly with its homeless ones and its householders inclines towards Nibbana, slopes towards Nibbana, flows towards Nibbana, reaches Nibbana.
-- Mahavacchagotta Sutta (Nanamoli/Bodhi trans.)
In MN 104, the Buddha explains to the brahmin Ganaka Moggallana why not all of his disciples attain Nibbana. He finishes, thus:
So too brahmin, Nibbana exists and the path leading to Nibbana exists and I am present as the guide. Yet when my disciples have been thus advised and instructed by me, some of them attain Nibbana, the ultimate goal, and some do not attain it.
-- Ganakamoggallana Sutta (Nanamoli/Bodhi trans.)
The Maggasamyutta contains the following instruction from the Buddha himself:
Bhikkhus, if wanderers of other sects ask you:
"For what purpose, friends, is the holy life lived under the ascetic Gotama?"
— being asked thus, you should answer them thus:
"It is, friends, for the abandoning of the fetters ... for the uprooting of the underlying tendencies ... for the full understanding of the course … for the destruction of the taints ... for the realization of the fruit of true knowledge and liberation ... for the sake of knowledge and vision ... for the sake of final Nibbāna without clinging that the holy life is lived under the Blessed One."
Then, bhikkhus, if the wanderers of other sects ask you:
"But, friends, is there a path, is there a way for attaining final Nibbāna without clinging?"
— being asked thus, you should answer them thus:
"There is a path, friends, there is a way for attaining final Nibbāna without clinging."
And what, bhikkhus, is that path, what is that way for attaining final Nibbāna without clinging? It is this Noble Eightfold Path; that is, right view ... right concentration. This is the path, this is the way for attaining final Nibbāna without clinging.
Being asked thus, bhikkhus, you should answer those wanderers of other sects in such a way.
And finally, the famous line declared by Buddhas and Arahants when reaching Nirvana, repeated through the entire sutta pitaka:
"Birth is destroyed, the holy life has been lived, what had to be done has been done, there is no more coming to any state of being".
Note: All emphasis in the quotes are my own doing.
No. If your goal is to attain enlightenment you've gotten it all wrong and I would recommend Brad Warner's book, Hardcore Zen. It is much more important to act appropriately.
From the book:
I'd been doing zazen for for over a decade by then and was pretty miffed that I had yet to reach enlightenment...
Let me give you a little background. In a nutshell, there are two major schools of Zen in Japan: Soto, to which Nishijima belonged, and ... my teacher Tim McCarthy had studied and taught; and the Rinzai school... The difference between them is this: the Rinzai shcool believes in enlightenment and the Soto school does not.
He goes on to explain why simply having the goal of attaining enlightenment will pretty guarantee you will not reach Nirvana. Attaining enlightenment is a selfish goal. The selfish can not, by definition, attain enlightenment. It is a catch-22 type paradox.
If you don't buy that, then at least you should be aware that is a school that not only does not have a goal of attaining enlightenment, but they don't even believe in it.
Many times, especially when responding to speculative questions, the Buddha announced that he teaches just two things — the universality of suffering and the path to the ending of suffering. That's what Buddhism is all about, no matter which of the many traditions. The Buddha taught the cause of suffering is clinging. This becomes a particular problem when one clings to the notion of a self or to a goal, such as the question implies.
One compounds the suffering if one announces to the world one is seeking the end of suffering. Thus the arising of the Mahayana, the first major division among the disciples of the Buddha. Mahayana pointed out the contradiction. They proposed an ideal of helping all others towards their ending of suffering, through compassion and generosity — the Bodhisattva ideal. Vajrayana derives from Mahayana and thus includes the Bodhisattva ideal.
Modern Theravada teachers frown on announcing whether one has become enlightened. Only those who had ended their concept of a self to be clung to, having achieved complete selflessness, would know whether the speaker had achieved complete selflessness. This takes the issue of Enlightenment off the table.
In the end all Buddhists seek the end of suffering. Some use the Bodhisattva vow to do it. Others seek it quietly. But all paths lead to the same end. At the end of the day, to be a Buddhist is to seek the ending of clinging.
Simple answer to your two questions
Is Nirvana a goal which every Buddhist must achieve? Yes. Ending the suffering is the ultimate goal of a true budhist
Is there any other things more important than attaining Nirvana? No
To answer this more, All budha's sutta and all teachings point to how to attain nibbana nothing else. In order to attain nibbana only thing you have to do is look into yourself and your sitha how thoughts come and go every second. Trick is not to attach to any of it because its impermanent. But if you get attached to these thoughts and feelings karma cycle begins in another words life begins at that moment.
Look at the Four stages of enlightenment and first step most important Sotapana.
The three fetters which the Sotāpanna eradicates are: (excerpted from Wikipedia for better explanation)
Self-view - The speculative view that a so-called self exists in the five aggregates (physical forms, feelings/sensations, perception, mental formations and consciousness) is eradicated because the Sotāpanna gains insight into the selfless nature of the aggregates.
Skeptical Doubt - Doubt about the Buddha, his teaching (Dhamma), and his community (Sangha) is eradicated because the Sotāpanna personally experiences the true nature of reality through insight, and this insight confirms the accuracy of the Buddha’s teaching.
Clinging to rites and rituals - Eradication of the view that one becomes pure simply through performing rituals (animal sacrifices, ablutions, chanting, etc.) or adhering to rigid moralism or relying on a god for non-causal delivery. Rites and rituals now function more to obscure, than to support the Right View of the Sotāpanna's now opened dharma eye.
The Sotāpanna realizes that deliverance can be won only through the practice of the Noble Eightfold Path.