"an amazing queen"
It's a little complicated, so take a breath, but its not that bad. There are three overlapping issues, but the first two basically only apply to "Standard Arabic", and not the spoken vernacular language. In the actual spoken language, it's a lot simpler.
Nouns and adjectives have a final vowel of short u/a/i to indicate, roughly, subject/object/"posessive" cases (in Standard Arabic ONLY, in dialect the vowel disappears). Adjectives should be in the same case and end in the same vowel as the noun they describe. Even though "an amazing queen" is not a full sentence, in Standard Arabic you should still choose the correct vowel to indicate the expected role and case of the noun in the sentence. In this instance, the "u" in "malika-t-u-n" and "mumtaaza-t-u-n", indicates that she is the subject of the implied sentence.
After the final vowel of a noun or adjective, "n" is added to show indefiniteness (in Standard Arabic only, for the most part), meaning "AN amazing queen" rather than "THE amazing queen". This is called tanwiin or nun-ation and is written by adding an extra stroke to the final vowel, rather then by using ن. This can never happen for words that start with "الـ" meaning "the" because that would be a contradiction. But it can happen with plural nouns, to mean "some". Adjectives must also have the same definiteness as the nouns the describe. So if the noun starts with "al", the adjective must also. And if alternatively the noun is indefinite and ends with "u/a/i+n", then the adjective must also.
The last letter, ة, taa marbuuTa, used only to form femenine endings of nouns and adjectives, and is normally pronounced simply as a short "a". However, it is pronounced "at" when the word is pronounced with tanwiin because it's indefinite, OR MORE IMPORTANTLY, when the noun or adjective describing the noun is connected to ANOTHER noun in "posessive" case that ends with "i", as in "the queen of the country", "malik-at al-balad-i". You DO NOT pronounce the "t" when the noun is followed by an adjective describing it, as in "the French queen", "al-malika-l-faransiyya". Then it's jist a short "a", malik-a.
And it's simpler in actual spoken dialects, which don't really use case endings or tanwiin amymore. So in Standard Arabic this sentence is read "malika-t-u-n mumtaaza-t-u-n", but the same sentence is read more simply as "malika mumtaaza" in most dialects. But point number 3, and everything about pronouncing ة as "at" when connecting feminine nouns and adjectives to other nouns still applies in dialects.
Also, if you can remember everything I said here, that's a significant portion of Arabic grammar. Nouns and the adjectives attached to them have vowel endings that indicate three cases that aren't necessarily pronounced anymore. All nouns are either definite (with "al") or indefinite (possibly with tanwiin). And when you put feminine nouns with other nouns (or use tanwiin), you pronounce the "t", otherwise it's a short "a" vowel.