Assuming that the event capacities are similar, I pulled the 2016 10k and half marathon finisher numbers from the weekends. That should give a decent approximation. Here's what I found:
10k Finishers
SWLS - 8,994
Tink - 8,369
DLH - 9,006
Super - 8,243
Half Finishers
SWLS - 11,612
Tink - 13,144
DLH - 13,084
Super - 7,734
The quick look shows that the 10k is probably pretty close to capacity for all weekends. The half finisher numbers, though, paint a pretty definitive picture that Superheroes is the least popular by a pretty large margin. Granted, this is only based on a single year's data, but it seems to bear out the conventional wisdom.