Straight up Stevia isn't too bad. It comes from the leaves of a plant and if anything is often sweeter tasting per volume than sugar. That doesn't mean people should be drinking soft drinks with it in great volume versus water, but it's an acceptable substitute IMO to sugar in coffee or the occasional drink and still better than cane sugar. Soft drinks contain plenty of other shit that's not particularly good for you in addition to the sweeteners too.

Sucralose is nasty stuff, and Splenda is just a brand, with their "sweetener" containing a bit of Sucralose in their chemical cocktail "sugar substitute" in addition to hefty doses of dextrose/maltodexrin as bulking agents.