In expert I would rate the boss difficulty from my experience all scaled to the time fought and amount of preparations I needed to achieve (a relatively) easy victory
Easiest to hardest: Golem(absolute joke of a boss requires zero preperation for the fight),
Destroyer(piercing weapons enough said ranked low due to the ability to summon the destroyer to make other fights easier,
King Slime(A decent challenge If encountered early prehardmode(pre EOC) usually by the slime rain),
Skeletron Prime(Hahaha build a few platforms and shoot it with your recently crafted Megashark results in a quick and easy fight),
Queen Bee(Only real threat are the bees and stingers but once you know her patterns she isn't too bad never died to her),
Brain of Cthulhu(While his illusions during the are interesting and all it is really easy to see through them making the fight no harder than normal mode) ,
Eater of Worlds(The vile spit adds complications that modify how you need to adjust your arena. Also the lack of a good spear kinda hurts the player against it),
Lunatic Cultist(the sheer options at the players disposal really reduces the difficulty substantially. The fight is still well done outside of the highly unsatisfying lack of drops
Eye of Cthulhu(matrix eye dodge really serves as a wake up call and the player is fairly limited in their options making it harder),
Wall of Flesh(The requirement of having to kill it very fast near the end) ,
Plantera(Took some real effort to down it and it has the tendency to take me with it...) ,
Duke Fishron(took a few deaths to get the pattern down for the 3rd form but it becomes an intense but relatively straight forward fight after that),
Twins(I swear Spasmasitism is my mortal nemesis)
Skeletron (with the limited options prehardmode this has got to be one of the toughest fights ever) Holds the record of over 50% of my deaths in expert mode
Moon Lord is Unranked (because I never bothered to finish the celestial event on my expert challenge, (3 pillars defeated(Vortex Solar and Stardust because of their gear), one left shield less) partly because I am worried about the fight, and partly because it allows easy access to the dungeon without accidentally triggering those annoying cultists like before
) *I am discounting any fights using moon lord drops fought with my "main" character in order to get its expert drops*