The anime one is my favourite. They added comedy and some extra detail to it, somewhat like K-on. The light novel's part was quite below my expectation though. When I picked up the book and read through the pictures, I was expecting something more funny and thrilling in that respect. I didn't read the manga to that point though. But from what I've read from the manga in the first few pages, I think the manga is missing too much detail, especially Taichi's sharp-tongue responses.
I agree that the anime have more comedy and some extra detail, therefore more interesting than the manga or LN.But, fanservice-wise, my favourite is the manga version. First, the art is simply beautiful (the best of the three). And second, it's Inaba.