How do you decide which cloud service model (IaaS, PaaS, or SaaS) is best for a business requirement?

When building out a cloud strategy, how do you decide whether IaaS, PaaS, or SaaS is the right fit for a project? Do you base it on factors like the level of control, complexity, or the speed of deployment needed? I’d love to hear from those who’ve made this decision—what were the key things you considered, and what worked best for your business?