"imi este dor de tine" or "mi-e dor de tine".....nice question...and good luck ;)

Or it can also mean: 'îmi lipseşti', but all three have the same meaning, so you can use either of them.

"Îmi lipseşti" means I miss you, but it's rarely used.
"Mi-e dor de tine" means I really miss you and it's more common.