How do you prioritize your work for clients?

I am a web developer and I’m currently struggling with prioritising my work for clients. The only way I know is first come, first served, so the clients who requested my services first get it first and so on, but I am not sure how to prioritise if the client on the first list is working on a smaller project and the clients later on are working on a bigger projects. Do I work on the bigger projects first and hope that the client with the bigger projects on the list doesn’t mind waiting longer? Or do I first finish my clients’ projects who so far has always paid on time and are not picky and only then work on the projects of clients who takes forever to pay me. What’s your approach if this is your work?