I'm certainly not an expert on all that but I think Stevia is a relatively newer one that is supposed to be less processed, but I have read it still can have a lot of side effects for some; headaches, GI issues, mood changes. I am not sure there the is enough longitudinal evidence on it to know if it can contribute to cancers or mental complications later on.
I mean if we're honest, our regular cane sugar is processed too, and has plenty of risks on its own. I've read a lot about how when companies took out fat from foods in the 70s and 80s people stopped eating them, so they added sugar to make them taste better, and found that sugar made people buy them even more and is very addictive. Maybe ironically, this is when Americans started having a huge increase in eating addictions and obesity.
I think the healthiest sweeteners are fruits and honey; in moderation of course. If you think about natural foods, that is what we are built for. We weren't really meant for diets with added sweetener in everything; that's just a ploy to get us hooked and buy more.