Torvald never explicitly says what he loves best about Nora. However, her certainly does imply certain things about his affection for her, particularly her beauty.
One thing we see him imply is that he loves her because he knows she loves him a great deal. We especially see this in Act 2 after Nora asks him for the second time not to dismiss Krogstad. Nora argues that it is narrow minded of Torvald to dismiss Krogstad simply because of a past indiscretion. She further argues that since Krogstrad writes for the newspaper, he may write something to slander Torvald, just like her father was slandered by the newspapers. While Torvald understands her worry about him being slandered, he is very angered at the thought of being called narrow minded. Nevertheless, he decides to forgive her because her anxiety proves how much she loves him, as we see in his speech to her:
My dear Nora, I can forgive the anxiety you are in, although really it is an insult to me ... It is an insult to think that I should be afraid of a starving quill-driver's vengeance? But I forgive you nevertheless, because it is such eloquent witness to your great love for me. (II)
Therefore, we can see from this speech that, even though he thinks she is very silly, he loves her because he sees how much she loves him.
However, his greatest expressed reason for loving her is her physical attraction. We especially see him praising her beauty in the final Act, particularly in his line, "Why shouldn't I not look at my dearest?--at all the beauty that is mine, all my very own?" (III). We especially see his passion for her in his speech describing his fantasies of her when they are out together at parties and further still in the feelings he describes he had as he watched her dance the Tarantella:
All this evening I have longed for nothing but you. When I watched the seductive figures of the Tarantella, my blood was on fire; I could endure it no longer, and that was why I brought you down so early--. (III)
Hence, we see that the real reason why Torvald loves Nora is because of his physical attraction for her, and the one thing he loves best is her beauty.
because she save him from his heathy by borroing money from krogstad