I might never get the exact answer for this.
I have that question alive in my head for 24 hours, since the first time I watched Casablanca. I watched again and still I cannot tell. Roger Ebert once said that "she paint his face with her eyes", and my... hers were a beautiful one. Do the mix look of longing and regret can be one of the indication? Or can I have a personal opinion that she loves Rick because the other man act so stiff I believe he cannot walk properly. But really, who Ilsa Lund really loves?