When I first started giving Lola her meals in treat dispensers, I got a basic treat ball from Pets At Home. The ball was okay, but there was no real way to change the difficulty. It was possible to change how big the opening was for treats to fall out--but I wasn't the only one who could change it! It took perhaps a fortnight, at most, for Lola to learn how to manipulate the opening so that it was as easy as possible.
After that, we moved on to a treat ball found at the local feed store. This ball had to be pulled apart to change the difficulty, so Lola couldn't make it easier on herself. There were two downsides to this ball though: it was difficult to entirely fill the ball, as the difficulty section divided the ball very crudely; and furthermore, the ball itself had a habit of sticking and having to be pried apart with brute force. Not good!
Recently, we got a Kong Wobbler. So far, it's great! Lola absolutely loves it. The wobbler twists apart, and has no way of changing difficulty so there's no way that she can cheat it. However--as said--there isn't a way to change the difficulty, beyond simply increasing the size of the food inside. That's the only downside, though; as it twists apart, it is very easy to clean (unlike the basic Pets At Home treat ball, as that could not be pulled apart).
If your dog likes to eat its meal from treat dispensers, go get a kong wobbler!