What is the Pillar of Islam?
According to the traditional understanding the pillar of Islam الْعِمَادِۙۖ is salah (prayer), jihad… Although the Quran doesn’t say what the pillar of Islam is. However, we can understand, that the pillar of Islam should be tawhid (unification or oneness of God). Because the only sin, which Allah will never forgive, is dying as a mushrik (associating anything with Allah. an-Nisa  48, 116). The opposite of shirk is tawhid (iman/faith).
To understand the second thing about this issue, is to look at the request of people in the hereafter, who wanted to come back to live. They wanted this to believe in Allah (al-Anam  27; Ibrahim  44; ash-Shuara  102) and doing amal salih/salihat (correctional acts):
“… Our Lord, we have seen and we have heard, so send us back, and we will act righteously نَعْمَلْ صَالِحًا we are now convinced.” (as-Sajda  12. Look also to Fatır  37; al-Muminun  99-100).
For the salvation in the hereafter, in tens of verses one more condition was requested: Amal salih (For example, al-Furqan  70; al-Maida  93; al-Talaq  11).
If we look closely, the things what people in the hereafter wanted to do are amal salih, not what the tradition tells us. Of course, every order or worship in the Quran is very important. It’s not possible, that a Muslim could underrate them or see them as unnecessary. However, it’s necessary to tell what the most important thing is, according to Allah.
The result is:
The pillar of Islam is believing in the form of tawhid (iman/faith) and its reflection: amal salih.