I've spent a lot of time thinking about this myself because I'm in a fairly unusual situation that I don't get a chance to throw rocks often. I live in a suburb and it's hard to find the space to make rocks safe.
So all my initial slings were built around tennis balls. But then I quickly realized they were not going to work well for smaller ammo like rocks or golf balls.
So then I went on a sling design vision quest to find the perfect middle ground that works for everything. And I found what just about everyone else has found. There really isn't any one perfect design

If you keep at it with different designs, you'll learn to be able to immediately see what sorts of ammo a certain design will be able to throw without even having to try it.
Anyways, I found my preferred general-use design is an overlapped leather strip pouch often called an aussie design. Not quite a split seatbelt pouch, but similar. Rather than a whipped connection to the cord, I prefer a spliced pouch connection so there's no change in stiffness. Anyways, you'll see this design in a lot of places like neandrthal, pan slings, etc.
More recently I've been using traditional split pouch braided slings, but I'm not quite sure I'm going to settle on those yet, and they're definitely not as flexible for different types of ammo(particularly smaller ammo like golf balls) as the overlapped pouch style.