I recently finished the game on expert, on a corruption world as a ranger build so I'll post my ranking. I had previously done an expert clear a few years that went as far as Plantera before I quit.
Eater of Worlds: I found this boss so easy that I temporarily wondered if it was bugged and he wasn't doing the proper damage.
Eye of Cthulhu: Felt like a pushover, make a half decent arena and bring some potions and be halfway decent at dodging.
Golem: I literally burst into laughter after I killed this boss because of how easy it was.
Wall of Flesh: Beenades are broken. Keep your distance and throw about 30 of them when the boss gets to about 20% hp and it's a free kill.
Brain of C'thulhu: I made a corruption and killed this guy mideway through pre-hardmode. He wasn't a total pushover, but I didn't bother to prepare as much as some of the other fights I did.
King Slime: On paper he's the easiest boss in the game. But if you fight him before EoC and don't try to cheese it, he is actually somewhat formidable, merely because of how weak your character is.
Destroyer: I feel like this would be a genuinely difficult fight if it weren't so damn cheesable. Sometimes I would get cocky while farming him and get hit by the head though... rip.
Queen Bee: I know some people find her one of the hard bosses, but I found her to be fairly easy. I didn't do anything special, just made a fair sized arena and brought potions.
Lunatic Cultist: Maybe it's a ranger thing, but I don't understand why so many people have trouble with this boss. I just stayed at range and shot things at him. I never even bothered with the clones, any dragons that spawned were dead before they reached me.
Skeletron Prime: First mech boss I killed, more by luck than by design. He's not a hard fight, but it's easy to make a mistake, get caught by the head and that's the end.
Deerclops: The fight is a little unintuitive, because usually against bosses you want to keep your distance, but once I figured out that staying close negates his most dangerous attacks I found it fairly easy.
Skeletron: I feel like he's the hardest pre-hardmode boss. You definitely want a good arena and as many potions as you can; still didn't have too much trouble.
Plantera: I felt very daunted going into this boss, because that is where I quit the game last time. However, I actually found the fight to be not that hard. She moves slowly, so in p1 you just do circles and in p2 you just keep your distance. Maybe it's a ranger thing, but I don't understand her reputation for being one of the hardest bosses in the game.
Queen Slime: I killed her after the mech bosses, and I didn't really understand the fight. There is so much to dodge and it's not easy; I pretty much brute forced it and killed her before I died. Feels like it would be a rough fight if you tried to kill her before the mech bosses.
The Twins: This is another boss I think that the difficulty has been overstated, but maybe it's harder with other builds. I just kept my distance and dodged as best I could. It feels like the fight actually gets easier once Spazmatism hits the second phase, because he goes slower.
Duke Fishron: One of the most notoriously difficult bosses in the game, but I killed him first go (it was a close thing mind you). His attack pattern is simple and he transitions to the final phase at such little health that the fight is pretty much over before he even teleports more than 2-3 times. His damage is high though, you can't fail at dodging. I killed him after Golem.
Moon Lord: I killed Moon Lord by accident using the Shrimpy Truffle + rain + infinite kite strategy. The fight feels too difficult if you don't attempt some sort of cheese, everything just does so much damage, you can't make mistakes.
Empress of Light: It took me probably 8 or so attempts to kill her, I struggled a lot. Ironically when I killed her I didn't even have potions, I just went for a practice attempt, because day was close and I was too lazy to go get more potions. It's a fun fight, but considerably more difficult than any other boss in the game except for Moon Lord.
To be honest, I think there is a tremendous gap in difficulty between the last three bosses in my list and all the other bosses, and a fairly dramatic gap between Duke Fishron and the other two.
As for the pillars:
Vortex - easy
Stardust - easy
Nebula - Moderate
Solar - Pain
I also found Dreadnautilus to be overtuned in my opinion, and I think phase 2 of the Martian Saucer's is ridiculous. The only boss I died more to than the Martian Saucer was Empress of Light.
It's been interesting watching people's opinions of the bosses and where there is disagreement.
