'Shared Universes' are big business nowadays with the Marvel Cinematic Universe being the most well known and successful. However, back in 2012 DC started to create their own shared universe using TV shows to build an ever increasing world of Superheroes.
I was late to the party but when I came back from travelling in 2015 I had a lot of time on my hands and so decided to catch up with all the series. As I navigated my way through the many, many episodes I kept a list to create this viewing order.
It can seem overwhelming since the Arrowverse now includes Arrow, The Flash, Vixen, Constatine, Supergirl, Legends of Tomorrow and Freedom Fighters: The Ray. It can even get confusing at times with Supergirl taking a place on a different Earth, The Flash creating alternative timelines and the Legends of Tomorrow constantly messing with time itself!
However, below is my Arrowverse Viewing Order as of 14/01/18.
Arrow Seasons 1 & 2 / Arrow: Blood Rush
•Arrow Season 1 (1 - 23)
•Arrow Season 2 (1 - 5)
•Arrow: Blood Rush (1 - 6)
•Arrow Season 2 (6 - 23)
Arrow Season 3 / The Flash Season 1 / Vixen Series 1
•Arrow 3×01: The Calm
•Arrow 3×02: Sara
•The Flash 1×01: Pilot
•The Flash 1×02: Fastest Man Alive
•The Flash 1×03: Things You Can’t Outrun
•Arrow 3×03: Corto Maltese
•The Flash 1×04: Going Rogue
•Arrow 3×04: The Magician
•Arrow 3×05: The Secret Origin of Felicity Smoak
•The Flash 1×05: Plastique
•Arrow 3×06: Guilty
•The Flash 1×06: The Flash Is Born
•Arrow 3×07: Draw Back Your Bow
•The Flash 1×07: Power Outage
•The Flash 1×08: Flash vs. Arrow
•Arrow 3×08: The Brave and the Bold
•The Flash 1×09: The Man in the Yellow Suit
•Arrow 3×09: The Climb
•The Flash 1×10: Revenge of the Rogues
•Arrow 3×10: Left Behind
•The Flash 1×11: The Sound and the Fury
•Arrow 3×11: Midnight City
•The Flash 1×12: Crazy for You
•Arrow 3×12: Uprising
•The Flash 1×13: The Nuclear Man
•The Flash 1×14: Fallout
•Arrow 3×13: Canaries
•Arrow 3×14: The Return
•Vixen 1x1
•Vixen 1x2
•Vixen 1x3
•Vixen 1x4
•Vixen 1x5
•Vixen 1x6
•Arrow 3×15: Nanda Parbat
•Arrow 3×16: The Offer
•The Flash 1×15: Out of Time
•The Flash 1×16: Rogue Time
•The Flash 1×17: Tricksters
•The Flash 1×18: All Star Team Up
•Arrow 3×17: Suicidal Tendencies
•Arrow 3×18: Public Enemy
•Arrow 3×19: Broken Arrow
•Arrow 3×20: The Fallen
•The Flash 1×19: Who Is Harrison Wells?
•The Flash 1×20: The Trap
•The Flash 1×21: Grodd Lives
•Arrow 3×21: Al Sah-him
•Arrow 3×22: This Is Your Sword
•The Flash 1×22: Rogue Air
•Arrow 3×23: My Name Is Oliver Queen
•The Flash 1×23: Fast Enough
Constantine Season 1
•Constantine Season 1 (1 - 13)
Arrow Season 4 / The Flash Season 2 / Legends of Tomorrow Season 1 / Supergirl Season 1 and 2 / Vixen Series 2 / The Flash: Chronicles of Cisco
•Arrow 4×01: Green Arrow
•The Flash 2×01: The Man Who Saved Central City
•The Flash 2×02: Flash of Two Worlds
•Arrow 4×02: The Candidate
•The Flash 2×03: Family of Rogues
•Arrow 4×03: Restoration
•Arrow 4×04: Beyond Redemption
•The Flash 2×04: The Fury of Firestorm
•The Flash 2×05: The Darkness and the Light
•Arrow 4×05: Haunted
•Arrow 4×06: Lost Souls
•The Flash 2×06: Enter Zoom
•Arrow 4×07: Brotherhood
•The Flash 2×07: Gorilla Warfare
•The Flash 2×08: Legends of Today
•Arrow 4×08: Legends of Yesterday
•Vixen 2x1
•Vixen 2x2
•Vixen 2x3
•Vixen 2x4
•Vixen 2x5
•Vixen 2x6
•Arrow 4×09: Dark Waters
•Arrow 4×10: Blood Debts
•The Flash 2×09: Running to Stand Still
•The Flash 2×10: Potential Energy
•Legends of Tomorrow 1x01: Pilot, Part 1
•Legends of Tomorrow 1x02: Pilot, Part 2
•Legends of Tomorrow 1x03: Blood Ties
•The Flash 2×11: The Reverse-Flash Returns
•Arrow 4×11: A.W.O.L
•The Flash 2x12: Fast Lane
•Arrow 4×12: Unchained
•Arrow 4×13: Sins of the Father
•The Flash 2x13: Welcome to Earth-2
•The Flash 2x14: Escape from Earth-2
•The Flash 2x15: King Shark
•Supergirl Season 1 (1 - 3, 5, 4 - 17)
•Legends of Tomorrow 1x04: White Knights
•Legends of Tomorrow 1x05: Fail-Safe
•Legends of Tomorrow 1x06: Star City 2046
•Arrow 4×14: Code of Silence
•Arrow 4×15: Taken
•Legends of Tomorrow 1x07: Marooned
•Legends of Tomorrow 1x08: Night of the Hawk
•Legends of Tomorrow 1x09: Left Behind
•The Flash 2x16: Trajectory
•Arrow 4×16: Broken Hearts
•The Flash 2x17: Flash Back
•Arrow 4×17: Beacon of Hope
•Arrow 4×18: Eleven-Fifty-Nine
•Arrow 4×19: Canary Cry
•Legends of Tomorrow 1x10: Progeny
•The Flash: Chronicles of Cisco
•Supergirl 1x18: Worlds Finest
•The Flash 2x18: Versus Zoom
•Supergirl 1x19: Myriad
•Supergirl 1x20: Better Angels
•Supergirl 2x01: The Adventures of Supergirl
•Legends of Tomorrow 1x11: The Magnificent Eight
•Legends of Tomorrow 1x12: Last Refuge
•The Flash 2x19: Back to Normal
•Supergirl 2x02: The Last Children of Krypton
•Supergirl 2x03: Welcome to Earth
•Legends of Tomorrow 1x13: Leviathan
•Legends of Tomorrow 1x14: River of Time
•Legends of Tomorrow 1x15: Destiny
•Supergirl 2x04: Survivors
•Arrow 4×20: Genesis
•Arrow 4×21: Monument Point
•Arrow 4×22: Lost in the Flood
•Supergirl 2x05: Crossfire
•Legends of Tomorrow 1x16: Legendary
•Arrow 4×23: Schism
•The Flash 2x20: Rupture
•The Flash 2x21: The Runaway Dinosaur
•The Flash 2x22: Invincible
•The Flash 2x23: The Race of His Life
Arrow Season 5 / The Flash Season 3 / Legends of Tomorrow Season 2 / Supergirl Season 2
•Supergirl 2x06: Changing
•The Flash 3x01: Flashpoint
•The Flash 3x02: Paradox
•The Flash 3x03: Magenta
•The Flash 3x04: The New Rogues
•The Flash 3x05: Monster
•The Flash 3x06: Shade
•The Flash 3x07: Killer Frost
•Supergirl 2x07: The Darkest Place
•Arrow 5×01: Legacy
•Arrow 5×02: The Recruits
•Legends of Tomorrow 2x01: Out of Time
•Legends of Tomorrow 2x02: The Justice Society of America
•Legends of Tomorrow 2x03: Shogun
•Arrow 5×03: A Matter of Trust
•Arrow 5×04: Penance
•Arrow 5×05: Human Target
•Legends of Tomorrow 2x04: Abominations
•Arrow 5×06: So It Begins
•Legends of Tomorrow 2x05: Compromised
•Arrow 5×07: Vigilante
•Legends of Tomorrow 2x06: Outlaw Country
•Supergirl 2x08: Medusa
•The Flash 3x08: Invasion!
•Arrow 5×08: Invasion!
•Legends of Tomorrow 2x07: Invasion!
•The Flash 3x09: The Present
•Arrow 5×09: What We Leave Behind
•Arrow 5×10: Who Are You?
•Legends of Tomorrow 2x08: The Chicago Way
•Legends of Tomorrow 2x09: Raiders of the Lost Art
•Legends of Tomorrow 2x10: The Legion of Doom
•Legends of Tomorrow 2x11: Turncoat
•Supergirl 2x09: Supergirl Lives
•The Flash 3x10: Borrowing Problems from the Future
•Supergirl 2x10: We Can Be Heroes
•The Flash 3x11: Dead or Alive
•Arrow 5×11: Second Chances
•Supergirl 2x11: The Martian Chronicles
•The Flash 3x12: Untouchable
•Arrow 5×12: Bratva
•Supergirl 2x12: Luthors
•Supergirl 2x13: Mr. & Mrs. Mxyzptlk
•Supergirl 2x14: Homecoming
•The Flash 3x13: Attack on Gorilla City
•The Flash 3x14: Attack on Central City
•Arrow 5×13: Spectre of the Gun
•Legends of Tomorrow 2x12: Camelot/3000
•Legends of Tomorrow 2x13: Land of the Lost
•Arrow 5×14: The Sin-Eater
•Arrow 5×15: Fighting Fire with Fire
•Supergirl 2x15: Exodus
•The Flash 3x15: The Wrath of Savitar
•The Flash 3x16: Into the Speed Force
•Legends of Tomorrow 2x14: Moonshot
•Arrow 5×16: Checkmate
•Supergirl 2x16: Star-Crossed
•The Flash 3x17: Duet
•Legends of Tomorrow 2x15: Fellowship of the Spear
•Arrow 5×17: Kapiushon
•Arrow 5×18: Disbanded
•Supergirl 2x17: Distant Sun
•The Flash 3x18: Abra Kadabra
•The Flash 3x19: The Once and Future Flash
•The Flash 3x20: I Know Who You Are
•The Flash 3x21: Cause and Effect
•The Flash 3x22: Infantino Street
•The Flash 3x23: Finish Line
•Arrow 5×19: Dangerous Liaisons
•Arrow 5×20: Underneath
•Legends of Tomorrow 2x16: Doomworld
•Legends of Tomorrow 2x17: Aruba
•Supergirl 2x18: Ace Reporter
•Supergirl 2x19: Alex
•Supergirl 2x20: City of Lost Children
•Supergirl 2x21: Resist
•Supergirl 2x22: Nevertheless, She Persisted
•Arrow 5×21: Honor Thy Fathers
•Arrow 5×22: Missing
•Arrow 5×23: Lian Yu
Arrow Season 6 / The Flash Season 4 / Legends of Tomorrow Season 3 / Supergirl Season 3 / Freedom Fighters: The Ray
•Legends of Tomorrow 3x01: Aruba-Con
•Arrow 6x01: Fallout
•Arrow 6x02: Tribute
•Arrow 6x03: Next of Kin
•Supergirl 3x01: Girl of Steel
•The Flash 4x01: The Flash Reborn
•The Flash 4x02: Mixed Signals
•Supergirl 3x02: Triggers
•Legends of Tomorrow 3x02: Freakshow
•Supergirl 3x03: Far From the Tree
•The Flash 4x03: Luck Be a Lady
•Legends of Tomorrow 3x03: Zari
•Supergirl 3x04: The Faithful
•The Flash 4x04: Elongated Journey Into Night
•Legends of Tomorrow 3x04: Phone Home
•Legends of Tomorrow 3x05: Return of the Mack
•Supergirl 3x05: Damage
•Supergirl 3x06: Midvale
•The Flash 4x05: Girls Night Out
•Arrow 6x04: Reversal
•Arrow 6x05: Deathstroke Returns
•Arrow 6x06: Promises Kept
•The Flash 4x06: When Harry Met Harry...
•The Flash 4x07: Therefore I Am
•Legends of Tomorrow 3x06: Helen Hunt
•Supergirl 3x07: Wake Up
•Legends of Tomorrow 3x07: Welcome to the Jungle
•Arrow 6x07: Thanksgiving
•Supergirl 3x08: Crisis on Earth-X, Part 1
•Arrow 6x08: Crisis on Earth-X, Part 2
•The Flash 4x08: Crisis on Earth-X, Part 3
•Legends 3x08: Crisis on Earth-X, Part 4
•Arrow 6x09: Irreconcilable Differences
•Supergirl 3x09: Reign
•The Flash 4x09: Don't Run
•Legends of Tomorrow 3x09: Beebo the God of War