The free tier on Oracle Cloud is nuts. 200GB block storage, 10gb object storage, 24GB RAM 4 core ARM VM, 2 x86 VM's 1GB RAM each. From my experience, it's also really hard to get charged since you have to upgrade your account from a free tier. I haven't been charged one cent in several years.

I don't know about the others, but AWS is not free and it's not cheap. There are ways to set alerts on budget overruns, but there is no foolproof way to limit monthly spending. Eventually you will configure something that goes out of control, or I was able to conceive of a few scenarios where a malicious third party could rack up charges for, e.g. S3 egress costs. Good luck contesting the charges more than once. I shut everything down and pulled out of AWS. I believe that Azure's free trial featured a hard cap, but still requires a credit card to open your account.