The phrase "best sling" refers to the top or highest quality type of sling available. It means that out of all the options, this particular sling is the most effective and reliable. Full definition
This sling makes it to the list of best slings for breastfeeding because it doesn't hurt the mom's back and is very easy to navigate even for a beginner mom. (abckidsinc.com)
Not intended for the Must Have Everything Mom, the MamaRoo is for the mom who knows that one good sling does it all! (mamaroobabysling.com)
After a few good slings over the bed I decided that they were secure, though they didn't sit flush on the wall because the glue was so thick. (domesticimperfection.com)