It strikes me strange to get this question--as if you'll need to make your own appreciations and feelings fit into categories of personal characteristics or behavior that a particular woman wants to hear, instead of being authentic expressions of what strikes you as wonderful, breathtaking, spellbinding, calming or alluring about her. Do you have the expectation that she needs or seeks compliments from you to make her feel that she is important enough to you? or beautiful enough? Or do you think she wants you to recognize what SHE wants you to value about her, because she does or wants to value those aspects of herself? (A feminist way to express this -- which could be appropriate or not depending partly on her age and on the sector of society and family and peergroup expectations she has grown up with -- would be Does she want to be perceived and admired as a woman performing in woman's roles with&for you as a man, that is as a partner and complement ("other half") or "object of your affections"--or as a Subject or Director of her own path in life. As one chosen to be loved, or as one choosing to love you as her equal, even though you're different from each other. (A lot depends on whether you've been together long enough to outgrow the instinctive "soulmated-partners" phase of marriage, from several to 10 or so years, and whether she conceives of herself as a ship with her own captain on bord, or as a passenger-seat rider in your car on the road of life.) So I'll try a few ideas, even though I don't think you're expecting anything like the way I think about love relationships; for my wife grew up in the beginnings of feminism in colleges, and I was in grad school when it started, I helped launch the Men's consciousness development, and I've lived with her for over 30 years.
What about the feelings you have when you spend time alone with her?--and doing what? If you love her personality, what are the aspects that light you up? If you love her movements and her body, what are the parts that make you admiring and/or hot? What do you love about her mind and how she uses it? Which of her interests fascinate you and show you aspects of reality you never understood until she showed them to you?
If you are asking this question because you think you need to respond as she would want to aspects of herself and her lifestyle and goals that she has chosen to develop and value, what are those choices, values and directions in life?
If you want your words about love to express your soul, what are soul-bearing experiences for you? (Or do you prefer to guess what SHE thinks soul is and try to meet her there--which would be fine if you haven't thought much about soul for yourself.) Here are some "soul-bearing" moments in life, as I know them: Silence in Nature; breathing together in lovers' embrace; spiritual moments in music; aesthetic beauty in arts; poetic understanding (when a meaning stretches out to surround you and be all you need to know in an extended moment); when Time stops through beauty, loving, eye contact, or appreciation of the web of life and the unity of much more than yourself and those nearest to you; the immensity of mountains and valleys, and that immensity in the one you love.
(That means that soul-bearing words may be difficult to make shareable, but soulful experiencing can be learned together.)
And by the way, if you should happen to think WHAT you would want to say you love about the woman you love if you knew she could die (of a terminal disease) in a year or less, YOU MIGHT HAVE A VERY EASY TIME COMING UP WITH SEVERAL HOURS AT LEAST OF ADMIRING and unhesitating praises to say, beginning with her toes and including her thoughts and everything you can remember that you have loved doing together with or seeing her do.
But that might be too hard for both of you to bear. So it's just our fate as humans that no matter when the question you've posed comes up, anybody in the situation is going to have too much or too little to say.
When I first met my wife, she told me she had never been too sure that she was pretty enough; so I told her I was going to love spending the rest of my life convincing her how pretty she was to me. And believe me, there's never been a dull minute when I seize a chance to do that again. But I wouldn't have wanted to hang with her if her mind weren't fascinating and a great gift.