I am attempting to defeat all of the bosses, to beat Terraria. This includes hard mode.

Who are the main bosses, throughout both regular and hard mode? Is there a particular preference I should take in defeating them?

It would definitely be worth considering multiple platforms, and the optional bosses, to make the answer more useful to other users.

One of the great things about Terraria is that the order of bosses that you defeat is pretty flexible. Some bosses are even completely optional as far as progression goes. That being said, most players will usually beat bosses in the following order:

Main Bosses

Pre-Hardmode

1. Eye of Cthulhu. Either this boss spawns on its own after a while, or you will spawn it yourself using a Suspicious-Looking Eye. The Eye has the least health of any boss, and has only three distinct attach patterns.




2. Eater of Worlds/Brain of Cthulhu (depending on Corrupted or Crimson world). Summon this boss by destroying three Shadow Orbs/Crimson Hearts.




3. Skeletron. He blocks the entrance to the dungeon. If you don't kill Skeletron, a Dungeon Guardian will spawn anytime you try to enter the dungeon. DG will one-shot you even if you have the best armor in the game.




4. Wall of Flesh. After throwing a voodoo guide doll into lava in the underworld, this boss will spawn. Beating WoF activates hardmode and allows you to summon new bosses.

[These bosses can be beaten in any order, but this is the order I've always done them in. Also, technically Eye of Cthulhu isn't required, but he shares the same ore drop as Eater of Worlds, and the ore is required to make one of the better swords in the game.]

Post-Hardmode

5. The Destroyer. A huge worm type boss, most people face him before any other mechanical boss because of easy access to several piercing weapons that make him really easy to beat (like Nimbus Rod or Daedalus Bow).




6. The Twins. Somewhat more difficult. Usually requires an aerial arena to beat.




7. Skeletron Prime. Pretty tough, since he has 4-5 attacks that he performs simultaneously. You better be good at dodging if you want to beat him.

[Note that the mechanical bosses can be beaten in any order.]

8. Plantera. Once all three mechanical bosses have been beaten at least once, Plantera Bulbs will spawn in the jungle. Break one to spawn this boss. Once Plantera is defeated, it drops a temple key (which can be used to open the Jungle temple) and allows more monster types to spawn in the dungeon.




9. Golem. This boss is found in the jungle temple and can be summoned by using a solar cell on the altar at the bottom of the temple. Be sure to disable any traps in the room first, otherwise the fight will get quite tough!




10. Lunatic Cultist. Once the Golem is defeated, several cultists will spawn near the dungeon, praying to some symbol. Kill these cultists to spawn the Lunatic Cultist, who absorbs the symbol's strength and fights you in the air. Beating the Lunatic Cultist will spawn four Lunar Pillars in your world.




11. Lunar Pillars. Not a boss per se, but still a challenging series of fights. Each pillar has its own set of enemies and obstacles. You'll know you're close to one when the colors on your screen start changing to the color of the pillar you're close to. The pillars do not despawn and remain on the map until destroyed.




12. Moon Lord. The hardest boss in the game (so far). Destroying the four Lunar Pillars will cause him to spawn after about 30 seconds to 1 minute (don't remember the exact timing). A definite challenge even with the best armor in the game. Getting help from friends is not required but recommended.

Optional Bosses

• King Slime. Hard to kill right away, but pretty easy once you've acquired some decent beginner gear. Spawns at the end of a slime rain event.




• Queen Bee. Occasionally in the jungle you'll find bee hives. Inside the hives is a 2x2 block that, once destroyed, spawns the Queen Bee. Drops various bee-related items when defeated.




• Goblin Invasion. An event in which a bunch of goblins will attack at once. Has a random chance of happening in the morning once the player has more than 200 health.




• Snowman Invasion. An event that happens when the player uses a snowglobe (available from presents that only drop during the annual Christmas event - see this question for more details on how to get it year-round). A bunch of snowmen will attack in waves.




• Pirate Invasion. An event where pirates attack in waves. Good source of money and gold furniture drops. Careful of the captain, he packs a punch!




• Duke Fishron. This boss will only spawn when you fish in the ocean while using a truffle worm as bait. He's fast, tough, and drops some really good gear. An arena is highly encouraged.




• Pumpkin Moon. A Halloween-themed invasion event where the player has to beat as many monsters in one night as possible. Getting to higher waves yields greater rewards, but also greater risk of death. The "end boss" of this event is the Pumpking.




• Frost Moon. A Christmas-themed invasion event where the player has to beat as many monsters in one night as possible. Getting to higher waves yields greater rewards, but also greater risk of death. The "end boss" of this event is the Frost Queen.




• Martian Madness. An invasion event where aliens attack. This event is triggered by letting a martian probe (found in upper atmosphere area) see you and escape.




• Old One's Army. A special Dungeon Defenders 2 crossover event that can be started by placing an Eternia crystal in a crystal stand (both purchased from the tavernkeep NPC). The difficulty of the event varies depending on how far in the game you've progressed. The "final boss" of this event is Betsy, though she only appears if you've beaten the Golem.

These are all bosses you have to fight, how to spawn them, and what you should do to fight them. Skeletron is not needed because mechanical skulls can rarely drop, although I would recommend defeating him.

Wall of Flesh: Spawned by dropping a guide voodoo doll into lava. You will need a guide to make it work. You should build a bridge across lava and break holes in the obsidian houses.

Destroyer: Spawned by using a mechanical worm. You should make a big box with campfires and maybe a heart lamp.

The Twins: Spawned with the mechanical eye. You should use the same box for all 3 mechanical bosses (Destroyer, Twins, and Skeletron Prime).

Skeletron Prime: Spawned with a mechanical skull.

Plantera: Spawned by breaking bulbs that appear in the jungle after defeating the mechanical bosses. mine out a large space around a bulb and put campfires and a heart lamp in it. Make shure the bulb isn't near the underworld or surface.

Golem: Spawned by using a lizard power cell at the lizard altar. To get there you use the key from plantera to get into the jungle temple. lizard power cells can be dropped by lizards or found in chests. Put walls next to the lizard altar because he can't go through walls. Also break all statues and traps in the room and make a wall so that other lizards can't come inside the room.

Lunatic Cultist: Spawned by killing the enemies that spawn outside the dungeon after defeating Golem. Make a platform at the dungeon or make a box around you because his attacks can't go through blocks except his phantasmal dragon.

Solar Pillar, Vortex Pillar, Stardust Pillar, and Nebula Pillar: These are sometimes not considered bosses, but I put them in my list. They spawn at different places in your world after defeating the Lunatic Cultist. They have a force field that will go away after defeating 100 enemies that spawn near it. A tip for crawltipedes: circle around them and hit their back where they can only take damage.

Moon Lord: Spawns 1 minute after defeating all 4 pillars. Make lines of platforms at your base and make a row a blocks for a shield for his phantasmal deathray. Put lots of healing things around the arena and use potions and food like pumkin pie. When I defeated him, I did it with 2 of my friends. You (and maybe your friends) should use full beetle armor and solar eruptions.

Here are links for how to craft the mechanical boss spawners:

https://terraria.gamepedia.com/Mechanical_Worm
https://terraria.gamepedia.com/Mechanical_Eye
https://terraria.gamepedia.com/Mechanical_Skull

Please up vote my answer!