I suggest a partially refined cane sugar like Turbinado, but it really depends on the wine and whether you want any flavor from the sweetener. High fructose corn syrup or normal white cane sugar would be fine for a neutral flavor.
I don't sweeten many wines (I like mine dry), but for sweetening most of them I just make a simple syrup with sugar and enough water to dissolve it. I've also used honey (great in crabapple wine), but my favorite way is one you have to plan in advance.
Save some of the original must, stick it in the freezer, and then use that to sweeten the finished wine- that's the original definition of "back sweeten"- using the original must to sweeten the wine. It's great for wines like blackberry, since it retains more of the fruit flavor and doesn't dilute the wine with any water.
I've had some luck using sucralose in watermelon wine. No need to add K-sorb. Dextrose has worked better for me, smoother, with Rosé and whites than Sucrose. I've tried beet sugar but never liked how it turned out. It's definitely dependent on the recipe. You could bench test maybe?
Depending on the RS desired in the bottling stage, i use a 1.5L empty bottle, half fill with the wine in question, add granulated sugar one cup at a time and stir with sanitized spare siphon rod until dissolved, then add to bucket prior to bottling.sweet white and reds 2 cups, semisweet 1 cup.