Code Geass. I prefer it to Death Note because of how the story plays out. Death Note also just was a bit boring after L died and I didn't like how they acted afterwards. Mello and Near were okay and all but it just seems that Light wasn't all into besting them. He just wanted them dead so they didn't get in his way. Also,the animation,the plot and the main characters from Code Geass are better. There's a bit more humor,less annoying characters and overall better plot structures.
If I had to choose Death Note. I like how it proved a series doesn't need a bunch of action or blood for a series to be really exciting and keep you on the edge of your seat. I just loves the way Death Note bent my mind and really made me think. Though, I love Code Geass as well~
Plot wise, I prefer Death Note because I just love how mind bending it got at times. It was an overall joy to watch for me and it still remains my favorite anime.
Character wise, I have to say I prefer Code Geass's. It's not that I dislike Death Note's characters, I really do love most of them, it's just that the Code Geass set of characters were more interesting to me.