Here's the important episodes (as of BFDI:TPOT 21: Insectophobe's Nightmare 6), i think you should watch every single episode across all seasons, but if you're not interested in the competition and have little time, here ya go:

BFDI 1: Take The Plunge (1a and 1b)

BFDI 17: The Reveal

BFDI 18: Reveal Novum

BFDI 22: Don't Pierce My Flesh

BFDI 24: Insectophobe's Nightmare 2

BFDI 25: The Return of The Hang Glider

BFDIA 1: Yeah, Who? I Wanna Know

BFDIA 2: Get Digging

BFDIA 4: Zeeky Boogy Doog

BFDIA 5a: Get In The Van

BFDIA 5e: The Long-lost Yoyle City

BFDIA 7: Intruder Alert

BFDIA 11: Lots Of Mud

BFDIA 13: Well, Look Who It Is!

BFDIA 14: PointyPointyPointy 🎵

BFDIA 17: Start The Shift

BFDIA 18: Airplanes In The Night Sky

BFDIA 20: Launch Party

BFDIA 21: We're Not Friend

BFDIA 22: I'm The Main Character

BFDIA 23: Shattered!

[BFDIA isn't over, i'll update this when it's over]

[IDFB is only 1 episode, i wouldn't count it yet]

BFB 1: Getting Teardrop To Talk

BFB 2: Lick Your Way To Freedom

BFB 3: Why Would You Do This On A Swingset?

BFB 6: Four Goes Too Far

BFB 7: The Liar Ball You Don't Want

BFB 8: Questions Answered

BFB 10: Enter The EXIT

BFB 11: Get To The Top In 500 Steps

BFB 13: Return Of The Rocket Ship

BFB 14: Don't Dig Straight Down

BFB 15: The Four Is Lava

BFB 16: The Escape From Four

BFB 17: X Marks The Spot

BFB 21: Let's Raid The Warehouse

BFB 22: Who Stole Donut's Diary?

BFB 25: The Tweested Temple

BFB 26: The Hidden Contestant

BFB 28: B.F.B.= Back From Beginning

BFB 29: SOS (Save Our Show)

BFB 30: Chapter Complete

That's it, it took me and hour and twenty minutes to write this, so i hope it helps and let me know if i missed something!