There is no such thing as free or equitable medical care anywhere in the world. It's a misnomer. There are aspects of socialized medicine that are good (I think). To some degree, you have a guarantee that your basic medical care will be covered. At a minimum, diabetics can get access to insulin in places like Canada and the UK, which is the very basic thing that keeps us alive for the short-term. BUT, those places have limits on things like test strips and if you want a pump or CGM, you have to jump through many hoops and can be waiting for a LONG time. In addition, I know in the UK the care you are able to receive varies depending on which county you live in. I think the same thing goes for Canada (i.e., different provinces/jurisdictions have different procedures and limits).
In the U.S., if you're really poor, you qualify for Medicaid, and you can actually get decent care through Medicaid, depending on what state you live in. All diabetic supplies are covered through Medicaid. If you're old and/or disabled, you may also qualify for Medicare. Medicare also has its limitations (I know pumps aren't covered for many and CGMs are definitely not covered), but again, you'll have basic access to insulin and some testing supplies.
Now, if you're lucky enough to have a job that offers decent health insurance in the U.S., you definitely have access to some of the best medical care around. I am fortunate enough to fall into that category and I have never had an issue getting what I need (pump, CGM, insulin, supplies, etc). Yes, I do spend about $200-$300 per month out-of-pocket on co-pays and miscellaneous stuff related to diabetes, but I am VERY lucky that I can afford this.
So, I don't think there is a "best" country for medical care and diabetes. It sort of depends on what your needs are. If you're a T1, best (to me) means having guaranteed access to insulin and the latest technology that one requires to manage their BGs and stay alive and complication free. And that doesn't really exist as far as I know.