My list here is mostly based off of my first fight with them so here it is.
1. King Slime: Big ol Slime that just hops around, no big deal. Expert added some more minions to the fight so that was neat.
2. Eater of Worlds: The segmented body that splits after some damage can be beaten pretty easily with a strong enough weapon by just standing still. Even on Expert he was still easy.
3. Brain of Cthulu: Pretty hard if you're stuck in the crimson cave area but if you have an arena set up he's pretty readable, even during expert.
4. Golem: Yeah he's the easiest hard mode boss, haven't fought his Expert form but I assume it's just as easy
5. Eye of Cthulu: Really easy on normal mode but the Expert one destroyed me and gave me a surprise with the sudden spike.
6. Queen Bee: Kinda hard on regular mode but if you're not stuck in a hive then she's otherwise pretty simple, Expert Mode made her into a mini-gun of a boss.
7. Destroyer: Like Eater this one is a big ol worn that has many segments, though these ones don't break apart so piercing weapons will have a field day, Expert kinda just beefed it up and made it hit harder but otherwise it's the same.
8. Skeletron: Regular mode is easy, but like Eye he has a spike in Expert Mode. I'd say the second hardest pre hard mode expert boss.
9. Skeletron Prime: This one is tough, though not as notable as the other two because it's still pretty similar to ol Skeletron, Expert is relatively the same.
10. Wall of Flesh: It's the final pre hard mode boss for a reason, this guy is a monster to fight especially with the location of the battle, Expert Mode version is still the same but that doesn't mean it isn't harder than regular mode, it's quite the challenge.
11. Lunatic Cultist: Ok this guy was a nice surprise, first fight with him was a mess, second time going back I noticed his patterns and gradually it became easier although he is an HP sponge. Haven't done expert for him yet sadly.
12. The Twins: THE hardest mech boss hands down, me thinking just because they were the mecha version of Eye of Cthulu made me challenge them first, and man that was a mistake. Expert for these two was relatively the same but they were always pretty hard to begin with so thats not saying much.
13. Plantera: Ooh boy, definety the hardest "required to progress" boss in the game. Especially with the location of the fight. I'm not the best at massive arenas so I usually have a small to medium sized arena. Sadly again, haven't done the Expert Mode of this fight. But I have done the 3ds version of it, and I think that might be even harder than the Expert version.
14. Duke Fishron: Love me some bonus bosses and the Duke is no exception. First fight with him ended as you would expect, I died a lot. But I figured it out sooner or later. By now I know his patterns like the back of my hand, because I wanted the Shrimpy Truffle so I fought his expert version before I even fought the Expert mechs. That was a mistake at first but eventually I did beat him and got my Truffle.
15. Moon Lord: Yeah he's the final boss for a reason so it's no surprise he'd be the hardest. It's not that his patterns aren't readable, but his attacks take up so much space and eat up health. I'll eventually get to his expert version but only after I get done farming the regular version for the Terrarian.
So yeah, there's my entire book series of a list, hope you enjoyed reading it.