You may not want to pick your favorite writing idea for your first novel. I’m not saying you can’t, but I would like to point out some reasons to consider working on something else.
9 times out of 10, your first book is not going to be very good. At least, it won’t be your best. If you think about it, that makes sense because you’ve never written one before. Any time you try something new, there’s a learning curve. You get better at it with practice.
It can be better to practice with something that you’re not extremely emotionally attached to. If you feel really strongly about a plot or a character, you may not want to change it at all. Even if that’s what the story needs. That’s one reason that writing classes give you specific prompts and may recommend that you work on something besides your favorite novel idea.
Working on something you don’t feel strongly about lets you focus on the writing aspect and gain experience. Once you have more experience, you may be able to work on your great idea with more flexibility. You also are less likely to get sick of it because it isn’t working out. If you have to abandon some prompt you found online that isn’t working, it’s much less of a disappointment than having to abandon an idea you really like.
And trying a different book first doesn’t mean that you can’t work on your idea at all. Character details and world-building are often part of the story that authors like best. They’re also good outlets for the creative urge to work on that story.
You may find that world-building is enough for you. Or you may prefer not to work on it at all for a while. Or you can decide that this strategy won’t work for you and try to write that idea now. That’s fine. This recommendation, like all advice about what to write, should be taken with a grain of salt and adjusted to fit you and how you work.
Even if you decide to work on your favorite idea for your first novel, you will practice writing, and, odds are, you will improve. My goal is to point out options and strategies you may not have considered.
Whatever you choose, I wish you the best of luck.