The problem with the Qur'an as a source is that it isn't really a narrative account. Even apparent references to historical events are often oblique and rely on the interpretation in the Tafsir, so that even if the Qur'an may refer to an actual event our knowledge of that event is only as reliable as the tafsir.

Surah al-Fil is a good example of this, where the Qur'an makes an obscure reference to "the companions of the Elephants". The traditional sources like the tafsir describe this as an invasion by one of the Ethiopian Kings of Himyar against Mecca which was repelled by the Meccans in the year of Muhammad's birth.

None of this is actually present in the text, and there are no other sources for these events. It may have occured, but that's dependent on the reliability of the Tafsir in interpreting the Quran, not the Quran itself. The tafsir, like the hadith and sira, were compiled more than a century after the Qur'an itself and depends on highly unreliable (academically speaking) oral transmission and accounts.

As such the Qur'an is perhaps most useful as a historical source simply as a physical, archaeological text. The discovery of early manuscripts and Qur'anic inscriptions, as well as the proliferation of the Qur'an, demonstrates things like 1. the historical existence of Muhammad 2. that he successfully created a large community of believers in Arabia at the turn of the 7th century, 3. that our current text is very nearly identical to the text that he would have preached.

These are critical historical conclusions that, had the Qur'an been passed down orally like the hadith, might be in dispute. As it is, there is a broad, even overwhelming, consensus about each of these points.

The hadith, including Bukhari's hadith collection, are far, far more problematic. There is no consensus in the academic community about their reliability. In general, however, the academic community is far more skeptical of these accounts than religious believers are. This has led some revisionists like Patricia Crone to totally reject these traditional religious oral accounts like the Hadith in favor of the contemporary documentary sources.

While many historians don't go so far as to reject these accounts outright, there is a strong preference for contemporary documentary sources given how often these seem to contradict traditional religious accounts and hadith collected more than a century after the death of the prophet.

Nonetheless, biographers of Muhammad who reject the traditional sources will struggle to find other material on which to write their histories. This has led to a longstanding trope in Islamic Studies of prefacing descriptions of the life of Muhammad with sentences like "We have no idea how reliable the traditional oral accounts of the life and sayings of the prophet may be, but they say this..." and then tell the traditional religious account.

So to answer your initial question the basic response is that the most reliable sources are contemporary documentary sources, typically in the form of papyri fragments, inscriptions found on rock or bone, and buildings or other archaeological evidence. Some of this can be indirect or fragmentary, so a partial fragment of a Qur'anic verse found in an inscription that is accurate to the current text would give credence to the idea that the text hasn't changed much, even if we only found a fragment. Likewise, even though we don't have any documentary evidence of the hijra, early inscriptions using hijri dating attest to the reliability of the date of hijra and its importance to the early community of believers.

The Qur'an and the constitution of Medina are two of the most important of these contemporary documents.

As far as historical reliability goes, while there is no consensus on these issues, the hadith are generally regarded as being potentially more reliable than the Sira or Tafsir, but in the eyes of many historians that may not be saying much.

For more on these topics I would suggest having a look at:

Discovering the Qur'an by Neal Robinson for the content of the Qur'an as well as some of these historiographical analysis questions.

Writing the Biography of the Prophet Muhammad: Problems and Solutions, which is an excellent and succinct examination of these problems. See also In God's Path by Robert Hoyland for what something of a "middle road" approach might look like. I haven't read it, but Hoyland's Seeing Islam as Other's Saw It is the most extensive collection of contemporary non-Muslim source materials.