The best idea is the one that
A) you are passionate about
B) people want
C) is scalable

See below for further discussion:

"A" is useful because it helps ensure that you'll stick with your startup over time and through difficulties, and that you have an above average knowledge of the field to which the idea is being applied. This means that you will know how to best pivot the idea during development if necessary (always is), and it's less likely that someone else has already thought of your idea and is doing it better.

For "B", you'll need to quantify this in some way specific to your product. You can start with the idea that you think has the most market potential, but you'll need to quickly find out if _you_ yourself can make something that people want. That means making some sort of prototype which you can show people and gauge interest on (by collecting interested emails, by having them take the time to test the product and give feedback, etc.).

"C" is also important because coming up with a good idea that people like, and coming up with an idea that would be good for a startup are two different things. If you don't have a business background, always talk to one or two people that do, in order to hash out whether the idea is scalable.