God's loyal angels are called "holy" and "elect."
"the Son of man also shall be ashamed of him, when he cometh in the glory of his Father with the holy angels" (Mark 8:38; cf. Rev. 14:10-12).
"I charge thee in the sight of God, and Christ Jesus, and the elect angels" (1 Tim. 5:21).
[both quotes are taken from the ASV]
Concerning 1 Tim. 5:21, Gordon Fee observes:
"The inclusion of the elect angels is unusual and serves to intensify the solemnity of the charge. Elect may either refer to the angels as the chosen ministers of God who carry out his will (Bernard) or serve as a contrast to the fallen angels (Kelly), probably the latter, given the context of judgment" (Fee, Gordon. 1 and 2 Timothy, Titus. P. 131).
When Jesus speaks of τῶν ἀγγέλων τῶν ἁγίων in Mark 8:38, he evidently means that the angels are "holy, set apart, sanctified, and consecrated" to God (Spiros Zodhiates). While we want to avoid what James Barr called "illegitimate totality transfer," I think that the aforementioned definitions for ἅγιος overlap with one another. The adnominal ἅγιος points to the faithfulness and sinlessness and consecrated status of God's loyal angels--their purity and sacredness in the sight of YHWH. This holiness is derived, however, and not self-caused like the holiness of God.
In the Similitudes, we also read:
"But you, having been strengthened by the holy Angel,and having obtained from Him such intercession, and not being slothful, why do not you ask of the Lord understanding, and receive it from Him?" (Sim. 5.4.2)
The ancient patristic writings thus refer to the angels as "holy."