But to each their own. I'll play along! And the answer is: You've got me! I can't find any bottles with built-in baffles to keep the water from sloshing around. Which isn't to say there aren't any, but I guess I'm a little skeptical. It seems to me that adding baffles would also add considerable cost to a low-price item, making it a hard sell. But if somebody out there can prove me wrong, I'm all ears.
Meanwhile I can offer the following helpful tips: "Soft" canteens, the ones used in hydration systems like those made by Camelbak or Platypus, collapse as they empty, eliminating most sloshing. It's also helpful to keep water bottles as low on your pack as possible. The top of a pack moves around a lot more than the lower portion, so store bottles in lower pockets. For years I've had on my Dana Designs Terraplane a device called a "Rib" that holds water bottle in an assembly that straps across my stomach. It also has a zippered pouch for Snickers bars, lip balm, film, and so on. Very little sloshing from that, although from now on I'll probably be very aware of it. So thanks a lot I'll probably be slosh-conscious for the rest of my natural life.