The face, or the eyes?
They work best in tandem.
On the assumption that the question wasn't a tease, try this: go into a website or consult a magazine where you find big cosmetic ads – even just the covers, in many cases – and conceal the face of the girl to reveal ony the eyes. You will often discover that you have just found a dead fish.
I think that a mouth can reveal far more emotion and intent
than do eyes.
Equally, if not ridden to death by post-production jockeys, it can work the other way: tight close-ups of eyes can be very powerful.
But there we touch on the problem to which I referred in the OP: models.
To do much in the manner of expression takes one of two things: a God-given skill and/or life-experience, of which the latter is the not-so-secret weapon of 'international' women in their mid-forties upwards. Maturity betrays youthful skin and freshness, but the years add their patina of sophistication which, of itself, has immense appeal to the more experienced adult.
Who wants to photograph teenagers?