Hey guys,
Let me see if I can help and shed some light here.
A muslim is a muslim by decalaration of Shahada that there is no god but the One God, Allah, free from all partners, and that Mohammed is his Messenger, his Apostle. He commits to the 5 pillars of Islam (Declaration of Belief, Daily prayers, fasting of Ramadan, Zakat of his capital, and pilgrimage to Mecca) and the six tenets of faith (Belief in God, his Angels, his Messengers, his Books, Judgement Day, and fate whether good or bad). Negating or declaring any of those to be false takes the person directly out of the fold of Islam and with unanimous agreement across the scholars, whomever does so is not a muslim. As brother Chuck before mentioned, the Khawarej (defectors) added Jihad as a pillar. Even though Jihad is one of the highest most glorious deeds one can perform under its proper conditions, making it a pillar is falsification of God's dictated religion and takes one out of its fold.
Not performing the pillars though out of neglect or laziness is not kufr, for example a person who does not fast out of weakness of will or lack of concern, but is a huge sinner. Prayer though is different. Majority of scholars believe that neglecting prayer is an act of kufr even if it is out of laziness, citing many hadiths such as "The difference between the believer and an infidel is leaving prayer" and "Between a person and kufr is neglect of prayer", as well as Quranic verses and other hadiths that say lack of prayer erases all deeds just like disbelief and that it corrupts the faith and religion completely. Other scholars say that it is not an apostacy (ridda) kind of kufr, but a smaller kufr which is "Kufr Bel-NeAAma", and they cite Hadiths and Quranic verses such as "God forgives all sins except Shirk (worshipping others besides God), and forgives whatever is beneath that" or "No person who said "There is no god but The God, Allah" will spend eternity in hellfire". The four Imams were split on that, Ibn Hanbal maintained that it is full apostacy, while Abu Haneefa maintained it is the lesser one. It is Abu Haneefa who also introduced the argument that if an apostate's repentence is to make a declaration (Shahada) and to declare the tenets of faith and recognize the pillars, then the neglector of prayer is already repented since he already made and maintains the Shahada and recognizes the tenets of faith and pillars of Islam. Malik and Shafie were for a while of the opinion like Abu Haneefa, but later changed and agreed with Ahmed Ibn Hanbal.
It's important to note that this disagreement is regarding the position of the person as a muslim, and whether or not he should be regarded, and therefore treated under an Islamic ruling, as an apostate. All scholars agree however that neglect of prayer corrupts all other deeds and is a complete eradicator of faith.
Now, with regards to the Quran, its fundamental characteristic dictated in Islam and proven is that it is infallible, and the prophet dictated that it will be preserved till Judgement Day and it is not for anyone to touch it or try to suggest adjustments or negate a ruling. Muslims believe all God's books including the Torat, Zabour, Sohof and the Bible, but we believe in their existence and correctness as they were sent at their time only. Muslims know they were modified, and their modification was confirmed by sources from within that religion as well as the Quran and the prophet -pbuh- and we are required not to take from them anything as per the incident when Umar Ibn El-Khattab came to the prophet -pbuh- and showed him a copy of the Torat and asked if he can read it and seek knowledge from it, and the prophet told him not to, and said that there will be falsehoods that he (Umar) might take as true, and truths that he might consider false, so he shouldn't seek anything in it for teaching or guideance (it is permissible of course to study them to understand Christian and jewish beliefs and be able to communicate and debate them). However, it is unanimous amongst scholars that negating or declaring false a part or or the whole of the Quran, would scar one's belief and take him out of the fold of Islam, since it denies the book's central dictated feature of being infallible and fully preserved.
Sunnah is slightly different however as per the asker. If a person denies a specific hadith or certain hadiths because he believes they are not authentic based on measurement or judgement of narration, it is not a sin at all. It is in fact what scholars do until today. Although the Bukhary and Moslem Sahih are regarded as the most accurate respected pieces of work Islam and humanity has seen, they are still works of normal humans, and questioning them is not a sin, even if it is based on poor work or judgement.
The sin comes when a hadith is neglected or denied based on pride, convenience or personal preference. As in some people who do not wish to talk about the prophet's marriage to Aisha at that age (out of lack of knowledge of the incident and its actual normalcy) would just say the hadith is not true so as not to bother arguing or defending. Or the hadith about a certain sunnah prayer because they don't want to commit to them. Pushing that further by arguing it with other muslims and non-muslims to try and establish it is Zandaqa, and is a huge sin for whomever does it carries its burden and the burden of every person who ever does it based on their talk. Equally any who support a disproven hadith out of pride or personal preference and later trying to push it to others as true. God forbid any of us to fall unwittingly in that, however it is still a major sin, not kufr.
Denying Sunnah and the need to follow any of it in entirity is, without doubt, an act that takes the person out of the fold of Islam. It scars the person's belief in the prophet's standing as the Apostle, and the foundation of direct orders within the Quran to follow the orders of the prophet. Additionally, denying Sunnah completely means that the methodology of fard prayer will be lacking, as according to the Quran for example only three fard prayers per day were literally mentioned and by that a person denying the Sunnay entirely will not pray Duhr and Maghreb, like the Quraaniin.
Hope this was informative and that I did not a head ache of such a long read for my first post.
And as always, God knows Best