Okay guys, I've made a rough chronological list of all the episodes with Flashbacks that happened before Twilight moved to Ponyville at Series's start.

We have to at least see some expository flashback, not just be told through dialog alone.

• • Hearth's Warming Eve (The Founding of Equestria shown via play)

  • The Legends of the Pillars (All the Legends of the Pillars from their respective episodes)

  • Twilight's Kingdom (Tirek's Imprisonment shown via book; assuming StarSwirl was still around at the time)

  • Shadow Play (Siren's Banishment and eventually Containment of the Pony of Shadows)

  • Princess Twilight Sparkle (Discord's Imprisonment)

  • Crystal Empire (Sombra's Banishment to Ice as the Empire vanishes into thin air)

  • Princess Twilight Sparkle (Twilight's seeing Nightmare Moon’s Banishment)

  • Friendship is Magic Pilot (The Book version of Nightmare Moon's Banishment)

  • Heartwarmings Tale (Canterlot at an unknown time, so I'm placing it here)

  • The Lost Treasure of Griffonstone (The History of Griffonstone unknown time ago)

  • Hooffields and McColts (The start of the HillFilly Feud)

  • Leap of Faith (Granny Smith's DareDevil Days)

  • Family Appreciation Day (Granny Smith's history with the Zap Apples)

  • A Friend in Deed (Cranky's Meeting and Missing Matilda)

  • Appleoosa’s Most Wanted (Trouble Shoes' backstory)

  • The Perfect Pear (Bright Mac and Pear Butter's story before the Siblings' birth)

  • Times they are a Changeling (Thorax's Hatching)

  • To Change a Changeling (Young Thorax and Pharynx)

  • Apple Family Reunion (A Flashback to a Reunion when A.J. was a baby) 

• • Canterlot Wedding (Filly Twilight's Foalsitter, Cadance)

  • Parental Glidance (Filly Rainbow's History with her Parents Perpetual Praising)

  • Games Pony Play (Filly Rainbow's Disappointment in Cloudsdale not hosting the Games)

  • Lost Treasure of Griffonstone (Filly Rainbow meets young Glida the Griffon)

  • Newbie Dash (Filly Rainbow gets nickname Rainbow Crash from a Trashcan Crashing)

• • The Crystalling (Starlight and Sunburst's Foalhood Friendship) 

  • Cutie Re-Mark (Sunburst gets his Mark and leaves Starlight)

  • Cutie Mark Chronicles (How the Mane 6 got their Cutie Mark on Sonic RainBoomsday as I call it)

  • Amending Fences (Filly Twilight's time in the Science; not sure if before or after her Mark is gotten)

  • Hurricane Fluttershy (Fluttershy can Hardly Fly)

  • Pinkie Pride (Cheese Sandwich's backstory with Filly Pinkie)

  • Mane Attraction (Filly A.J. and RaRa at Camp)

  • Where the Apple Lies (Filly A.J. and Colt Big Mac's Lie at the Hospital)

  • Cart Before the Pony (Filly Rarity losing to Derpy)

  • For Whom the Sweetie Belle Tolls (Sweetie Belle's 5th Birthday party)

  • Celestial Advice (Celestia's Realization that she has to let Twilight go to Ponyville)

I know it's not perfect, but like I said, it's a rough list. I'll be able to improve it once the 2nd half of season 7 adds to Netflix and it will be easier to view and check my speculation. I'd be happy to hear any comments about this, as long as it's kept civil and not just "Your Timeline is Wrong" This is just how I piece it out on my own.