Board Thread:New on Terraria Wiki/@comment-24136903-20131106094355/@comment-5406744-20140311040910

i believe:

the destroyer is easiest in damage wise [with the right equiptment] [his health is also Very Friggen high: 60000 to be exact.

the twins are easiest in movement, low defence and attack [^]

skeletron prime is hardest because he's and "all-rounder" kind of boss, his meele is tank, his ranged is strong and his health/defence is high.