I've got to go with manga. There are way too many incredible anime that have come from normal-style manga to ignore.

However, visual novels and, surprisingly, 4-koma manga aren't that far behind. Though a lot of visual novels are fluff, Welcome to the NHK gives us an idea of the depth that visual novels can bring to the table. The amount of anime made from 4-koma manga would probably surprise a lot of people, as well. Yuyushiki, Azumanga Daioh, Monthly Girls' Nozaki-kun, Kill Me Baby, K-ON, and Aiura are some of my favorites.

I'll go with Light Novels with Manga following closely after

A lot of good ideas come from Light Novels that can't even be shown in anime so I think that's best
4 Komas are nice, but more often than not, the anime feels really disconnected with constant jumps because 4 komas are usually made to be short but sweet events that can't really be long enough in an anime to flow too well (though there are exceptions)

I'm going Manga. Manga adaptions don't usually go badly while LN adaptions.... well, we can get some like NGNL but on the other spectrum we get mahou sensou and Mahouka..... so yea going manga.

It looks like my preferred source material for anime adaptions are LNs/Novels followed closely by manga then original works ...

It's a shame how the only good anime adaptions from VNs are Steins;Gate and Higurashi since they have as much potential to have just as greatly distinguished and enthralling plotlines like light novels ...

Most of my favorite anime are adapted from manga, so I have to choose that, even if the odds are a bit stacked with how many of them there are. I've enjoyed quite a few anime based on novels, but I find the transition isn't as clean typically. Which makes sense...since sacrifices have to be made to move from text to animation.

If we're talking purely of the source material and not the adaptations...my answer is still the same, though I tend to gravitate towards shounen and shoujo. I've only read a few seinen...and even fewer josei series.

Manga all the way. Most of the stuff I love either originates from manga and is portrayed best in manga form (deadman wonderland and tokyo ghoul are good examples since the Mangas are amazing, but the animes can't replicate it to be nearly as good)
Next would be visual novels, although their anime adaption are usually crap, the novels themselves are amazing.

Most of the Awesome Animes I have watched were from Manga!

Not me! I prefer Sayaka Ohara! 

As for poll, light novels to be safe from Shoujo... 

Gotta go Fast!

Phantom World has to be the best terrible anime I've seen in a while. Good lord, seeing MC jump on Mai's giant ass killed me.

Erased wrecked me slightly less this week. I'm extremely wary of sensei being the killer...but I feel like that's way too obvious at the same time. I'll wait and see how it's executed either way; obvious endings aren't always bad ones.