I think there's a subtle difference in how you interpret this question that makes a big difference here... It asks what your favorite medium for sci-fi is, but the answer could be different if you look at it in terms of in which medium your favorite sci-fi exists. If a medium has 99.9% crap, but a small sliver of shining excellence, then my favorite sci-fi might come from that medium, but I wouldn't call it my favorite medium because the average quality is horrible and I'm not expecting the next thing I see there to be any good. That's kind of how I see movies and TV: a few examples are really good, and I think that superlative things *could* be done in the medium, but on average it's generally meh.
There's also a question of volume. There are a lot more sci-fi books than there are sci-fi TV shows, and more TV shows than there are movies. I could concievably watch every single sci-fi movie made in a year. Probably the same with TV shows. I don't think I'd have any chance at reading all the sci-fi published in a year. So if I whittle down what I watch and read by whatever recommendation methods I can bring to bear, and those methods are effective, then maybe I watch the top 10% of movies and TV, and read the top 0.1% of books, so the average quality of what I watch is in the ballpark of the 95th percentile of quality, while every book I read is in the 99th percentile of quality. Thus my perception will be that books are of a higher quality, even if the reality is that there's just a lot more crap that I haven't read.
But the real difference to me is something that I realized not too long ago, and it's something inherent to the medium itself. With books most of all (and to a lesser extent games), I am in control of the rate at which I'm pushing information into my brain. The rate at which I can absorb the important aspects of a story isn't just different from person to person, it's different for me from day to day. If I'm reading, I can keep going at whatever pace my brain can keep up with, and if I hit a spot where I say to myself "woah, wait, what just happened?", I can skip back and re-read (which is also possible in movies and TV watched at home, not so much in theaters.) So this means that for a movie or TV show, if its information density rate isn't just exactly right for me on the day I'm watching it, then it'll either be going too fast and I'll feel lost, or too slow and I'll feel bored. The way those media counteract that is to make sure the information rate is slow enough that most people don't get lost, and then fill the gaps with content that is reasonably interesting, but relatively information-free. That can make for some shows that are fun and entertaining, but end up feeling a little empty. Of course, the counter to that is when you use content as a framework for your playground, and then create your own information density with the help of friends, acquaintances, and eventually strangers with common interests. Thus emerges fandom.