Angels holy and unholy

There are at least three references to holy angels in the New Testament.

Mark 8:38  If anyone is ashamed of me and my words in this adulterous and sinful generation, the Son of Man will be ashamed of them when he comes in his Father's glory with the holy angels.

Luke 9:26  Whoever is ashamed of me and my words, the Son of Man will be ashamed of them when he comes in his glory and in the glory of the Father and of the holy angels. 

Revelation 14:10  ... they, too, will drink the wine of God's fury, which has been poured full strength into the cup of his wrath. They will be tormented with burning sulfur in the presence of the holy angels and of the Lamb.

By implication fallen, non-elect angels are not holy. The holy angels, Gill notes, will be judges and will condemn unbelievers to everlasting burnings and pronounce the sentence on them. They will gather them out and seperate them from the righteous, cast them into the furnace of fire, will be spectators of their punishment and rejoice at it. The sight of their power and glory will increase the torment of the sufferers.

Angels reveal prophetic truth from God

 This comes out i Daniel 10 as well as in other places such as Ezekiel, Zechariah and Revelation.

Angels are referred to together as a heavenly host

Psalm 103:20, 21 Praise the LORD, you his angels, you mighty ones who do his bidding, who obey his word. Praise the LORD, all his heavenly hosts, you his servants who do his will.

Psalm 148:2  Praise him, all his angels; praise him, all his heavenly hosts.

The idea of hosts is also attached to armies and to stars. The angels form a great army, an army of light. (Cf Job 38:7; Revelation 9:1, 11).

Angels are not the only heavenly creatures, there are also seraphim

Isaiah 6:1-6 In the year that King Uzziah died, I saw the Lord, high and exalted, seated on a throne; and the train of his robe filled the temple. Above him were seraphim, each with six wings: With two wings they covered their faces, with two they covered their feet, and with two they were flying. And they were calling to one another: "Holy, holy, holy is the LORD Almighty; the whole earth is full of his glory." At the sound of their voices the doorposts and thresholds shook and the temple was filled with smoke. "Woe to me!" I cried. "I am ruined! For I am a man of unclean lips, and I live among a people of unclean lips, and my eyes have seen the King, the LORD Almighty." Then one of the seraphim flew to me with a live coal in his hand, which he had taken with tongs from the altar. 

(See also Revelation 4:7, 8)

Seraphim (flaming ones) may be the same as cherubim but they may be a different rank of creature to angels or cherubim.

Angels are to be honoured but not worshipped

Revelation 19:10 At this I fell at his feet to worship him. But he said to me, "Don't do that! I am a fellow servant with you and with your brothers and sisters who hold to the testimony of Jesus. Worship God! For it is the Spirit of prophecy who bears testimony to Jesus." 

Angels are wonderful creatures and it is tempting, if one sees them in all their glory, to worship them but they are not to be worshipped. Only God is to be worshipped.

Angels are capable of swift flight

Daniel 9:21 ... while I was still in prayer, Gabriel, the man I had seen in the earlier vision, came to me in swift flight about the time of the evening sacrifice.

Swift flight is no doubt a metaphor of some sort, a theriomorph of some description. What it identifies is the ability of Gabriel and other angels, no doubt, to travel speedily. Without the aid of machines or other helps of that sort they are able to cover huge distances in a short time.
This would seem to fit in with
Revelation 14:6 Then I saw another angel flying in midair, and he had the eternal gospel to proclaim to those who live on the earth - to every nation, tribe, language and people. 

Also Psalm 104:4 (quoted in Hebrews 1:14) He makes winds his messengers, flames of fire his servants.

Angels are always in God's immediate presence

Luke 12 8, 9 I tell you, whoever publicly acknowledges me before others, the Son of Man will also acknowledge before the angels of God. But whoever disowns me before others will be disowned before the angels of God.

Here the repeated phrase before the angels of God is another way of speaking of the immediate presence of God (see the parallel in Matthew 10:32, 33). We learn, therefore, with regard to angels, that they are always in God's immediate presence. What we also learn, however, is that believers ought to always be aware of an invisible world, the world of angels that is the world that really counts. So often we are aware of what men see and what they think but more important is what the angels see and what they think. When tempted to deny the Lord, think of the angels.
Revelation 3:5 is parallel in part
The one who is victorious will ... be dressed in white. I will never blot out the name of that person from the book of life, but will acknowledge that name before my Father and his angels.Also relevant is Luke 15:10, which we tend to think of as referring to angels but is really referring to God himself
In the same way, I tell you, there is rejoicing in the presence of the angels of God over one sinner who repents.
In Luke 1:19 Gabriel reveals that he stands always in the presence of God.