After seeing several requests, I have compiled the complete list of all the Adventure Time episodes that contain major plot, important backstory, major characterization, and/or Bubbline moments. I have also included optional episodes that I think will increase your understanding/enjoyment of the show (I put the reason next to it; if it says something about being an arc, backstory, characterization, etc., be warned that not watching may make the major plot points of the show a bit more confusing for you). 

I condensed the show down to 153 of the 10-minute episodes (91 episodes if you don’t watch any of the optional episodes). Optional episodes are in italics. Episodes with Bubbline moments, characterization of Marcy/Princess Bubblegum (PB), or major backstory/plot related to Marcy/PB are in bold. Anything with a an asterisk (*) can be watched at any time (order doesn’t matter, though you may still want to watch it in the same season).

I hope this helps someone out since I spent hours putting it together! Without further ado…

Season 1

1 x 2 “Trouble in Lumpy Space” | Introduces LSP

1 x 3 “Prisoners of Love” | Introduces the Ice King

1 x 5 “The Enchiridion”

1 x 9 “My Two Favorite People” | Introduces Jake & Lady Rainacorn (LR) arc

1 x 10 “Memories of Boom Boom Mountain”

1 x 12 “Evicted”

1 x 15 “What is Life?” | Introduces NEPTR + Ice King characterization

1 x 16 “Ocean of Fear” | Finn characterization

1 x 17 “When Wedding Bells Thaw” | Ice King characterization

1 x 20 “Freak City” | Introduces Magic Man

1 x 22 “Henchman”

1 x 24 “What Have You Done?” | Ice King characterization (*)

1 x 25 “His Hero”

Season 2

2 x 1 “It Came from the Nightosphere”

2 x 2 “The Eyes” | Ice King characterization

2 x 3 “Loyalty to the King” | Ice King characterization

2 x 6 “Slow Love” | Just a funny/cute episode (*)

2 x 7 “Power Animal” | Establishes more of Adventure Time universe

2 x 10 “To Cut a Woman’s Hair”

2 x 11 “Chamber of Frozen Blades” | Ice King characterization (*)

2 x 12 “Her Parents” | Jake and Lady Rainacorn (LR) arc

2 x 15 “The Real You”

2 x 17 “Death in Bloom” | Establishes more of the AT universe

2 x 18 “Susan Strong” | Introduces Susan Strong arc

2 x 20 “Go with Me” | Subtle Bubbline

2 x 24 “Mortal Folly”

2 x 25 “Mortal Recoil”

2 x 26 “Heat Signature”

Season 3

3 x 3 “Memory of a Memory”

3 x 5 “Too Young”

3 x 6 “The Monster” | LSP characterization/general cuteness (*)

3 x 7 “Still” | Ice King characterization

3 x 9 “Fiona and Cake” | Gender bending episode (*)

3 x 10 “What was Missing”

3 x 14 “Beautopia” | Susan Strong arc continues

3 x 17 “Thank You” | Not vital to plot, but adorable (*)

3 x 18 “Apple Thief” | Not vital to plot, but a favorite (*)

3 x 19 “Holly Jolly Secrets Pt. 1”

3 X 20 “Holly Jolly Secrets Pt. 2″

3 x 21 “Marceline’s Closet”

3 x 25 “Dad’s Dungeon” | Finn and Jake background

3 x 26 “Incendium” | Flame Princess Arc

Season 4

4 x 1 “Hot to the Touch” | Flame Princess arc

4 x 2 “Five Short Graybles” | Established more of AT universe (*)

4 x 5 “Return to the Nightsophere”

4 x 6 “Daddy’s Little Monster”

4 x 8 “Hug Wolf” | Generally cute (*)

4 x 9 “Princess Monster Wife” | Ice King characterization + great episode

4 x 10 Goliad

4 x 11 “Beyond this Earthly Realm” | Ice King characterization

4 x 13 “Princess Cookie” | Generally cute

4 x 15 “Sons of Mars” | Established more of AT universe

4 x 16 “Burning Low” | Flame princess arc + PB characterization

4 x 18 “King Worm” | Serious foreshadowing

4 x 19 “Lady and Peebles”

4 x 20 “You Made Me”

4 x 22 “Ignition Point” | Flame Princess Arc

4 x 24 “Reign of Gunters”

4 x 24 “I Remember You"

4 x 26 “The Lich”

Season 5

5 x 1 “Finn the Human”

5 x 2 “Jake the Dog”

5 x 6 “Jake the Dad” | Jake/LR arc

5 x 5 “All the Little People” | Cool and very slight Bubbline cameo

5 x 9 “All Your Fault”

5 x 11 “Bad Little Boy” | Gender bending + Marcy/Ice King development

5 x 12 “Vault of Bones” | Flame Princess arc

5 x 14 “Simon and Marcy”

5 x 16 “Puhoy” | Flame Princess arc

5 x 17 “BMO Lost” | SO ADORABLE. If you don’t watch you’re missing out (*)

5 x 18 “Princess Potluck” | Ice King characterization (*)

5 x 20 “Shh!” | Established more AT universe and is cute/neat (*)

5 x 21 “The Suitor”

5 x 22 “The Party’s Over, Isla de Senorita” | Ice King character development

5 x 23 “One Last Job” | Jake character development

5 x 26 “Wizards Only, Fools”

5 x 28 “Be More” | BMO backstory (*)

5 x 29 “Sky Witch”

5 x 30 “Frost and Fire” | Flame Princess arc

5 x 31 “Too Old”

5 x 32 “Earth and Water”

5 x 33 “Time Sandwich”

5 x 34 “The Vault"

5 x 38 “Red Starved”

5 x 40 “Play Date” | Finn character development

5 x 41 “The Pit” | Finn character development

5 x 45 “Blade of Grass”

5 x 46 “Rattleballs” | Finn character development

5 x 47 “The Red Throne” | Flame Princess arc

5 x 48 “Betty”

5 x 50 “Lemonhope Pt. 1″

5 x 51 “Lemonhope Pt. 2″

5 x 52 “Billy’s Bucket List”

Season 6

6 x 1 “Wake Up”

6 x 2 “Escape from the Citadel”

6 x 4 “The Tower” | Finn character development

6 x 6 “Breezy”

6 x 9 “The Prince Who Wanted Everything | Gender bending (*)

6 x 10 “Something Big”

6 x 12 “Ocarina” | Jake character development

6 x 14 “Princess Day”

6 x 16 “Joshua and Margaret Investigate”

6 x 19 “Is That You?” | Flame Princess arc

6 x 22 “The Cooler” | Flame Princess arc

6 x 23 “The Pajama War”

6 x 24 “Evergreen”

6 x 25 “Astral Plane” | Martin arc

6 x 26 “Gold Stars” | Lich arc

6 x 27 “The Visitor” | Martin arc

6 x 28 “The Mountain” | Flame Princess arc + Lemongrab characterization

6 x 29 “Dark Purple” | Susan Strong arc

6 x 32 “Friends Forever” | Ice King characterization

6 x 35  “Graybles 1000+” | Foreshadowing

6 x 38 “You Forgot Your Floaties”

6 x 39 “Be Sweet” | Lich arc + LSP character development

6 x 40 “Orgalorg”

6 x 42 “Hot Diggity Doom”

6 x 43 “The Comet”

Season 7

7 x 1 “Bonnie & Neddy”

7 x 2 “Varmits”

7 x 4 “Mama Said” | Finn/Jake character development

7 x 5 “Football” | Adorable BMO (*)

7 x 6 “Marceline the Vampire Queen”

7 x 7 “Everything Stays”

7 x 8 “Vamps About”

7 x 9 “The Empress Eyes”

7 x 10 “May I Come In?”

7 x 11 “Take Her Back”

7 x 12 “Checkmate”

7 x 13 “The Dark Cloud”

7 x 14 “The More You Moe, The Moe You Know (Part I)” | BMO arc

7 x 15 “The More you Moe, The Moe You Know (Part II)” | BMO arc

7 x 18 “President Porpoise is Missing!” | Adorable + Marcy cameo

7 x 21 “King’s Ransom”

7 x 25 “Flute Spell” | Introduces Huntress Wizard

7 x 26 “The Thin Yellow Line”

7 x 27 “Broke His Crown”

7 x 28 “Don’t Look”

7 x 29 “Beyond the Grotto” | Established more AT universe/cool (*)

7 x 30 “Lady Rainacorn of the Crystal Dimension” | LR backstory (*)

7 x 31 “I am a Sword”

7 x 32 “Bun Bun” | Finn + Flame Princess arc and characterization

7 x 33 “Normal Man”

7 x 34 “Elemental”

7 x 35 “Five Short Tables” | Gender bending (*)

7 x 36 “The Music Hole”

7 x 38 “Preboot”

7 x 39 “Reboot”

Season 8

8 x 1 “Two Swords”

8 x 2 “Do No Harm”

8 x 3 “Wheels” | Jake characterization/background (*)

8 x 4 “High Strangeness”

8 x 6 “Jelly Beans Have Powers”

8 x 7 “The Invitation”

8 x 8 “Whipple the Happy Dragon”

8 x 9 “Mysterious Island”

8 x 10 “Imaginary Resources”

8 x 11 “Hide and Seek”

8 x 12 “Min and Marty”

8 x 13 “Helpers”

8 x 14 “The Light Cloud”

Season 9

9 x 02 “Skyhooks”

9 x 03 “Bespoken For”

9 x 04 “Winter Light”

9 x 05 “Cloudy”

9 x 06 “Slime Central”

9 x 07 “Happy Warrior”

9 x 08 “Hero Heart”

9 x 09 “Skyhooks II”

9 x 10 “Abstract”

9 x 11 “Ketchup” 

9 x 12 “Fiona and Cake and Fiona” | Gender bending (*)

9 x 13 “Whispers”

9 x 14 “Three Buckets”

Season 10

10 x 01 “The Wild Hunt”

10 x 02 “Always BMO Closing” | Cute BMO episode that is sort of important to this season’s plot but you will still understand what’s going on without it

10 x 03 “Son of Rap Bear” | Flame Princess arc fulfilled; fun episode 

10 x 04 “Bonnibel Bubblegum”

10 x 05 “Seventeen”

10 x 07 “Marcy and Hunson”

10 x 08 “The First Investigation”

10 x 09 “Blenanas” | Finn characterization; brief appearance of PB

10 x 10 “Jake the Starchild”

10 x 11 “Temple of Mars”

10 x 12 “Gumbaldia”

10 x 13 “Come Along with Me”

Avatar Korra

Plaster figures showing women’s bodies as they would have appeared had they fit into Western clothing during four points in fashion history (1875, 1904, 1913, and the 1920s), designed by Bernard Rudofsky and sculpted by Costantino Nivola, as presented in the exhibition Are Clothes Modern? at The Museum of Modern Art, New York.