That's a tricky one. I'm a big fan of Bourbons, but since they're not restricted by copyright or branding, every UK supermarket and many other confectionery companies each make their own and the quality can vary quite a bit.

There are also two common form factors. The original is roughly 2:1 (as pictured in the above article) and the other, which is less common, is closer to 3:2 to match the common form factor of the Custard cream.

The supermarket own-brand I usually get (2:1) has the bonus of being dairy-free, which is not always the case.

Sometimes I catch myself involuntarily doing a little happy dance on biting into one, so the quality of that particular brand has yet to drop beyond that threshold for me.