Accessability is by far Skeletron, but with everything else in mind it's Queen bee. Resourcess are rather easy to get once you've gotten ahold of a honey bucket but the fight in itself is harder than any other fight since it doesn't follow an exact pattern, and it requires a fair bit of moving around for dodging, not to mention that unless you've made or found a proper room for an arena, she's prone to just avoid your damage by flying through blocks and what not, effectivly being tankier than one could tell from just raw numbers.

But heck, that's just what I think!