You look wonderful tonight.
STA-i be-NIS-si-mo sta-SE-ra — stress on 'sta-i', 'nis-', and 'se-'. 'Benissimo' is five syllables.
Complimenting your date's appearance at the start of the evening. 'Stare' refers to how someone looks in the moment, making it more personal than 'essere bella/o'.
'Stai' is the second person singular of 'stare' (to stay/to be). When used with adjectives like 'bene' or 'benissimo', it means 'you look'. 'Benissimo' is the superlative of 'bene' (well). This construction is specific to appearance in a given moment.
Quel vestito ti sta benissimo.
That dress suits you perfectly.
More specific — complimenting the outfit choice shows attention to detail
Sei bellissimo/a.
You are beautiful.
Uses 'essere' — a more absolute statement about beauty rather than tonight's look
Mi piace come ti sei vestita/o.
I like how you've dressed.
Acknowledges the effort made — Italians dress carefully for dates and appreciate this being noticed
Italians dress very deliberately for dates — there is a concept called 'bella figura' (making a good impression through appearance) that is central to Italian culture. Commenting positively on how someone looks is not superficial but shows you have noticed their effort and care.