if a girl were telling me that, cute, attractive and hot would be the only acceptable answers lol. It would porobably depends on his personality, you don't want to say anything that's going to make him blush and be shy... then again you don't want him to get conceeded and become all cocky. If you know him well, you'll be able to justify what the right thing to say is... maybe telling him how you feel would be a better step than telling him what you think of his physical appearence
The word itself doesn't matter as much as how you actually say it. Any of those words will work. Just be sure to smile, -seductively- smile might I add, when you say it. Deepen your voice a little to give it some effect. And if you're really bold, might want to say it close to him as well.
I think you could be more creative than cute, hot, attractive. How about you look like masterpiece scuplted by the greatest artists of the world, you look better than the Statue of David and Brad Pitt all wrapped into one.