Here's some breakdowns you might find interesting:
/Regeneration is the most popular secondary, with 41625 characters having this secondary.
Fire Blast/ is the most popular Primary set, with 30219 carrying it as their primary.
The most popular powerset of all, regardless of being primary or secondary, is Willpower, with 47913.
The four most popular powersets (either primary or secondary) of all are:
Willpower - 47913
Fiery Aura - 44945
Posted up a searchable and sort-able table of all of the primary & secondary powers along with the percentage of people that took them relative to the number of people that have the power set (all data from level 50 characters only):