Iblees was definitely not one of the angels. This is indicated by three things: the clear texts of the Quran, the physical attributes of Iblees and the attitude of Iblees.  

1- With regard to the clear statements of the Quran, Allah says (interpretation of the meaning): 

“And (remember) when We said to the angels: ‘Prostrate yourselves unto Adam.’ So they prostrated themselves except Iblees (Satan). He was one of the jinn.” [18:50] 

Al-Hasan al-Basri said: Iblees was never one of the angels, not even for an instant. He is the father of the jinn, just as Adam (peace be upon him) is the father of mankind. 

This was narrated by al-Tabari with a saheeh isnad (authentic chain of narration) as stated by Ibn Kathir in his exegesis. 

2 – With regard to his physical attributes, Allah stated that He created him from fire. Allah says (interpretation of the meaning): 

“He created man (Adam) from sounding clay like the clay of pottery.

And the jinn He created from a smokeless flame of fire.” [55:14, 15] 

It was narrated in Saheeh Muslim from the hadeeth (report) of ‘Aishah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) said: “The angels were created from light, the jinn were created from smokeless fire, and Adam was created from that which has been described to you.” 

So it is clear that there is a difference between the physical nature of the angels and that of Iblees, and he is definitely not one of them. 

3 – With regard to his attitude, Iblees disobeyed Allah by not prostrating to Adam, but we know from the Quran that it is impossible for the angels to disobey Allah. Allah says (interpretation of the meaning): 

“…angels stern (and) severe, who disobey not, (from executing) the commands they receive from Allah, but do that which they are commanded.” [66:6] 

Some reports which are not saheeh (authentic) have been narrated from some of the salaf (pious predecessors), suggesting that Iblees was the best of the angels, that he was one of the keepers of Paradise, etc. Imam Ibn Kathir commented on this by saying: 

Many reports to this effect have been narrated from the salaf, but most of them come from the Israiliyyat (reports narrated from Jewish sources) which were narrated to be examined. Allah knows best about the true status of many of them. There are some which can be rejected out of hand because they go against the truth that we have before us. In the Quran there is sufficient information so that we have no need for earlier reports, because hardly any of them are free from alterations, additions and subtractions, and many things have been fabricated in them. The earlier nations did not have scholars to examine these reports and eliminate those which are not sound, as this ummah (nation) has with its imams (religious leaders) and scholars who have great knowledge and are pious and righteous, who wrote down the hadeeth and examined them and showed what is sound and what is not, and what is fabricated, identifying the fabricators, liars and those narrators who were unknown, and other categories of narrators. All of this was to protect the status of the Prophet, the seal of the Prophets and the leader of mankind (peace and blessings of Allah be upon him), lest any lie be attributed to him and lest anything be attributed to him that he did not say or do. May Allah be pleased with them and make them pleased and make the Paradise of al-Firdaws their abode. 

Tafseer al-Quran il-‘Azeem.

And Allah knows best.