I recently had a great exchange with dataviz luminary Salma Sultana about where best to locate slice labels in a pie chart. The consensus that we came to was that it depends on the lengths of the category names and whether or not there are any very small slices.

If the category names are short and there are no very small slices, placing the labels inside slices or outside works well:

If, however, the category names are long and/or there are very small slices, placing the labels outside of the slices tends to work better:

BTW, European friends:
I'll be in the Benelux and Nordics regions in May and am looking to add one or two more workshops to my trip. Interested in an in-person dataviz and/or dashboard design workshop for your team, or in hosting an open-registration workshop in your city? Contact me for deets.