What is the hottest part of a chili/chilli/chile pepper?
The Wikipedia article on Capsicum reads, citing a FAQ on a page of the ollege of Agriculture and Home Economics of the New Mexico State University:


  Capsaicin is present in large quantities in the placental tissue (which holds the seeds), the internal membranes and, to a lesser extent, the other fleshy parts of the fruits of plants in the genus Capsicum. Contrary to popular belief, the seeds themselves do not produce any capsaicin, although the highest concentration of capsaicin can be found in the white pith around the seeds.


Cisneros-Pineda et al. measured via gas chromatography the content of capsaicin and dihydrocapsaicin in the pericarp, placenta, and seeds of seven cultivars of chili peppers and the results confirm this:



For dihydrocapsaicin, the difference is not so large, but the placenta still has a higher concentration.

In terms of concentration considering fresh mass the difference won't be as dramatic because while the seeds lose around half their mass by drying, the placenta will lose 90%.