Wow, so few manga supporters in this thread. There are a few animes that I much prefer to watch in anime form over manga, but in general I prefer manga strongly.
In anime, there are so many things that can go wrong to ruin the show. If everything is perfect except for the music, a few voice actors, or something else anime-specific, it could ruin the whole show. In the manga, your mind is what decides what's the best music for it, the best voices for the character. It's the most perfect it could possibly be, because you created it in your mind to be the best for you reading the manga. In addition, I've never been able to find a website that has as much free anime as the websites I know of with free manga scans. Some styles can only be shown in manga (yonkoma), and manga in general often comes before the anime, so the anime is just adapting the original and I'd rather have the original than the adaptation if I have a choice. There's also the matter of my attention span. I will never just watch a TV show; I'll watch a show and play Pokemon and chat with friends, etc. If I'm reading a manga, online or in my hand, it's easy to glance away from it and do other things. If I'm watching an anime, I'd have to pause it every single time, and it takes longer than a few seconds to watch the previous scene and remember where I was. Often the time it takes to get myself back into the anime is longer than the time I spent away from it. Finally, I can read manga anywhere. I don't need sound. If I don't have my headphones, that's fine. If where I am is crowded and I can't hear even with headphones, that's fine. I can still read manga. Anime needs a very specific place to be able to be watched, while manga is much more flexible.
tl;dr: manga manga manga