You're a treasure.
sei un te-SO-ro — stress on 'te-' and 'so-'. 'Tesoro' has three syllables: te-SO-ro.
A warm and affectionate expression — 'tesoro' is used as a term of endearment both as a compliment and as a pet name.
'Sei un tesoro' = you are a treasure. 'Tesoro' = treasure. In Italian, 'tesoro' is also widely used as a term of endearment equivalent to 'darling' or 'sweetheart'. Calling someone 'un tesoro' says they are rare, valuable, and precious.
Sei meraviglioso/a.
You're wonderful.
From 'meraviglia' (wonder/marvel) — a deeply felt expression of admiration
Sei un dono.
You're a gift.
'Dono' = gift — implies the person is a blessing in your life
Sei tutto quello che si potrebbe desiderare.
You're everything one could wish for.
Superlative — nothing more could be desired
Italian uses diminutives and terms of endearment ('tesoro', 'caro', 'amore') more freely and earlier in relationships than many northern European cultures. Being called 'tesoro' is warm and Italian — it does not necessarily imply deep commitment, but it does imply genuine affection and care.