It was a toss-up between Ken and Hyde.
Ken has got to be the strongest composer of the group, having written most of the 'epic' (dramatic, climatic, etc.) songs like Forbidden Lover, Jojoushi, tsuioku no jookei, kaze no yukue, Butterfly's Sleep, etc. And also, it goes without saying that his solos are the highlights of each song. Each solo fits tightly with each song, and each have their unique flair.
Hyde has also written a lot of great songs, like Ophelia and Anemone, but he tends to alternate between the poppier, commercially successful sounds of Honey (which I tend not to like) and the epic sounds of those ones mentioned here. In addition, his vocals are strong and easily distinguishable from other musicians. Even the soundalikes who have sprung up in the past don't have the same quality of intonation and variation that he has.
I ended up going for Hyde, simply because I liked his solo works better, even though I know we're talking about in the context of L'arc en Ciel. ROENTGEN showed that he can do soft, flowing, rich compositions, while I couldn't really get into Ken's SOAP works. If I had looked only at L'arc en Ciel stuff, I probably would have picked Ken.
In person, they both seem cool. However, Ken has a rather high-pitched voice that I would have never expected before hearing him speak. It got on my nerves (yeah, I'm superficial). Also, Hyde has this kind of reserved nature backstage sometimes, that I find a little endearing.