Formal:
* Ineffable: This word means "too great or extreme to be expressed or described in words."
* Unspeakable: Similar to "ineffable," this word emphasizes the impossibility of putting the feeling into words.
* Unutterable: Another synonym for "ineffable," highlighting the lack of words to describe the experience.
Informal:
* Speechless: This word emphasizes the inability to speak due to shock, awe, or emotion.
* Beyond words: This phrase is a common idiom used to express the inadequacy of language to convey the feeling.
* Words fail me: Another common phrase, implying that the speaker is unable to find the right words.
Literary:
* Mute: This word conveys a sense of silent awe or despair.
* Taciturn: This word describes someone who is habitually silent or reserved.
Ultimately, the best word will depend on the specific context and the nuance you want to convey.