You are irresistible.
sei ir-re-sis-TI-bi-le — stress on 'ti-'. 'Irresistibile' has six syllables: ir-re-sis-TI-bi-le.
A bold and direct flirtatious compliment — used when there is clear mutual interest and you want to express it unmistakably.
'Sei' = you are. 'Irresistibile' = irresistible — the adjective is the same for both genders (it ends in '-e', not '-o'/'-a'). This is a strong word — using it signals clear romantic interest.
Non riesco a resistere al tuo fascino.
I can't resist your charm.
'Fascino' = charm, allure — a sophisticated Italian word for magnetism
Ogni volta che ti vedo mi perdo.
Every time I see you I lose myself.
Romantic and slightly poetic — 'perdersi' in someone is a beautiful expression
Sei impossibile da ignorare.
You are impossible to ignore.
States the effect without fully naming the feeling — intriguing
Italian flirting often involves a playful cat-and-mouse dynamic — compliment, slight protest, teasing comeback. The reply 'sei terribile' (you're terrible) accompanied by a smile is actually a positive sign — it means the flirting is landing well.