I think that I fell in love with this character soon after Tsukushi described him as having marble-like eyes. 
Truly Hanazawa Rui is an amazing looking anime character. I love his short hair cut, and I especially like that one episode where he had his hair cut before he and Tsukushi went on a date (lucky girl!!). Many Hana Yori Dango fans are against Rui, but I think he is ten times better than Doumyouji (sorry Doumyouji fans!). Rui has this frank (and often short and rude) "nothing-matters" attitude that I find highly appealing, not to mention his soft and sexy voice (Kouji Yamamoto is an excellent seiyuu for Rui). I even like that Rui is sort of a lazy person (not unlike myself), and you can see him sleeping quite often in the anime. My favorite scene where Rui is sleeping is when Doumyouji barges into Rui's house and demands he wake up so Doumyouji can show Rui the cookies Tsukushi made for him.
Of, course, my number one reason that I love Hanazawa Rui is that he plays a violin. I adore violins, so you know that I'm going to love a cute guy that plays one. I used to listen to Rui's theme (the song he either plays or is playing during times where Tsukushi is watching him) every day--dai suki! I even bought the Hana Yori Dango CD, Hana Dan, just so I could listen to the full version and have a clear version of it on my computer. Despite being a lazy bum, Rui is quite athletic; he can play volleyball and basketball pretty well. He's not too bad with a razor either--he did a pretty good job of touching up Tsukushi's bad hair cut. Oh, and another of my top reasons I am in love with Rui is because he can be evil sometimes. For example (one of my favorite Rui scenes), Rui did not tell Doumyouji that he didn't sleep with Tsukushi until Doumyouji was on his flight just because of that bear incident. That rocked!! And I loved what Rui did in the very last episode--that was wrong of him, but I loved it. Without a doubt, Rui is not only on the list of my top best-looking guys, but also on my top list of favorite anime characters.
Ne, did I mention that Rui can also play the piano and the saxophone?? Aishiteru, Rui-sama! ^_^