i disagree on the k2's...skulls just mean badass, aka for the seths, he is sick, so skulls+seth=sick...
and for the hellbents, come on they are called the HELLbents afterall...so why not have some gnarly skulls on them? i think they are actually some of the sickest graphics i have seen anyways...
i personally hate the "graphics" of the original head mojos...so blain and boring...but more recently i would have to say all of elans twin tips...look horrible...