"Where do you get your ideas from?" is another question I'm often asked. It's a hard one to answer. I can get an idea from a newspaper article I've read or heard on the evening news. It might be a car passing me on the freeway driven by a hooded character, or a bum at the exit with a sign "will work for food." Even a TV show, documentary or movie can get my juices flowing.
Zap! This is where creativity gets launched. I start thinking..."what if?"
My best advice to you is to have pencil and paper handy, or if you're on the road a great deal, how about a hand-held recorder. Can the smart phones do that? Later, I type them up and put them in a file. When I'm in between novels, I can get out the file and thumb through my old ideas--some I had completely forgotten. One might tickle my fancy and launch my next journey.
Which of my ten novels is my favorite? That's like asking a mother which is her favorite child. Of course, I can answer that one with ease since I only had one son. Definitely, he is my favorite.
Historicals (and I would imagine that space and futuristic plots might fall in this category) are the most difficult to write. Your facts must be accurate. You can't kill someone with a firearm that had yet to be invented; or have mail delivered to your castle before the era of Benjamin Franklin; or mow the grass before the invention of the rotary lawn mower. You think....well, it's fiction, I can do whatever I want. Yes, you can, but the whole idea of writing a story is to make it believable. You want to take your reader on a journey and glaring errors snap them out of the mood that you've been trying to create.