I tend to like manga more because of the art and (usually) more detailed plot line. I also much prefer being able to use my imagination to picture a scene, rather than having it layed out in front of me. Novels without pictures do this best, but manga still allows some sort of imagination with just still images. Anime and film on the other hand, leaves next to no room for your own imagination.
That said, I watch anime a lot more often, simply because it's easier to watch whilst doing other things (usually talking on MSN) I also prefer to have reading items actually in my hand, but seeing as I don't have any money to buy things with I can't afford to buy manga, so reading it on the computer screen takes away from the experience a bit I think.
Although sometimes anime can be a lot more artistic, the only example I can think of is Bakemonogatari, as I haven't really watched that many series. So anime that is more creative, and a bit different from all the other series, gets my vote for being the best. But in general, manga > anime.