1: Goblin Army - Actually pretty difficult if you haven't min-maxed, Goblin Archers are a pain and they're all relatively tanky, worth it for Tinkerer though.
2: King Slime
3: Eye of Cthulu - It always spawns before King Slime for me, but I find it obnoxious to instantly kill two of the limited number of pre-Hardmode bosses not long after starting, so I save and quit every time it spawns so I can "build up to" the fight and actually feel some progression.
4: Brain of Cthulu/Eater of Worlds - I fight both in one world for the novelty of it, but not at the same time.
5: OOA Tier 1 - If my gear isn't clearing waves effectively enough I put off this tier until post-Skeletron.
6: Queen Bee
7: Skeletron
8: Wall of Flesh
9: Pirate Invasion - About as difficult as the Goblin Army was at the start of pre-Hardmode. I like to make a box around the Dutchman, pour some lava in it, then sit directly below in a safebox while leaving the game on to collect Greedy Ring components.
10: Solar Eclipse - I cheat a bit by hoiking into the Jungle Temple to get enough Eclipse summons to get the Bat Wings.
11: Dreadnautilus - I know it isn't actually a boss, but considering its health-pool and that I consider it the single most difficult enemy in the game relative to when you're "supposed" to fight it, it may as well be. Well worth the pain for the Sanguine Staff though.
12: Queen Slime - Actually harder than any of the Mech Bosses by a mile, even with the exact same gear. I only fight her first because I'm trying to vaguely stick to progression. Doesn't drop anything amazing though.
13: Twins - I consider them the most difficult Mech, but they're also the quickest to kill. For that reason I use them to get my initial stack of Hallowed Bars.
14: OOA Tier 2 - This is nowhere near easy right after the first Mech, but it's feasible and finally lets me replace my light pet.
15: Destroyer - OOA 2 gear can make this easier, sentries tend to be effective since the Destroyer has such a large hitbox.
16: Skeletron Prime - Takes ages to kill so I put it off for a while.
17: Plantera
18: Pumpkin Moon - I don't care how far I get in this as long as I kill both mini-bosses once and get the Hexxed Branch and Witch's Broom.
19: Frost Moon - I don't care how far I get in this either, as long as I kill each mini-boss once.
20: Empress of Light - Takes ages to kill with pre-Golem gear but it's doable. I use her night drops to fight the daytime version immediately after too. Terraprisma makes all future trash mobs a non-threat.
21: Golem
22: Martian Madness - Dangerous event, but has decent drops and Terraprisma takes care of ground mobs.
23: OOA Tier 3 - The toughest tier, but Terraprisma again makes it much easier than it would otherwise be.
23: Duke Fishron - Goes down pretty quickly if you fight him at this point, but it can also take you out quickly if you slip up. Excellent drops as well.
24: Lunatic Cultist
25: Lunar Events - Whatever pillar corresponds to the class I'm playing first, then in whatever order I feel like.
26: Moon Lord