I'm that friend who gushes about character growth, and Eustace is one of my favorite redemption arcs in 'The Voyage of the Dawn Treader'. If you want lines that hit the heart, I lean on these paraphrased moments (they're not verbatim, but they're what stuck with me):
'I'm a changed person' — the feeling after his dragon chapter when he finally understands himself. It’s short but huge: pride and selfishness take a beating, and you can practically feel his shame turn into humility.
'Forgive me' (to Aslan, in spirit) — Eustace's apology and willingness to be honest about his faults is so rare in kid characters; that humility is the whole point. Also, I love the bit where he admits he was wrong about others and about himself; it’s quiet but massive.
For fans who like scenes over soundbites, the dragon-waking and the getting-out-of-dragon-skin moment are where the best lines live. Re-reading those pages with a warm drink makes the lines land even harder — they’re little nails in the coffin of his arrogance, and it's oddly satisfying. If you’re making a fan-quote wall, mix one of those reflective lines with a line that shows his later humor and loyalty; his voice after change is sweeter and a lot more human.