I'm not really sure you can make such a cut-and-dried distinction. With Supernatural, even the episodes which might be termed MotW usually have an important ongoing-character-arc aspect to them, and the nature of the monster often relates to whatever emotional issues the boys are dealing with at the moment. In fact, with them both being such Men, who tend to keep their emotions bottled up anyway (Dean more than Sam), the monster is often the only way of getting to that emotional core at all. So what I'm saying is that even if an episode is not mythology-plot-arc heavy, it might still be important-character-arc heavy, and indeed sometimes the two are indistinguishable.
Having said all that, here are what I think are the most important episodes you would need to see:
Season 1
101 - Pilot (aka "Woman in White")
104 - "Phantom Traveller"
109 - "Home"
111 - "Scarecrow"
112 - "Faith"
114 - "Nightmare"
116 - "Shadow"
118 - "Something Wicked"
120 - "Dean Man's Blood"
121 - "Salvation"
122 - "Devil's Trap"
Season 2
201 - "In My Time of Dying"
203 - "Bloodlust"
205 - "Simon Said"
208 - "Crossroad Blues"
209 - "Croatoan"
210 - "Hunted"
214 - "Born Under a Bad Sign"
217 - "Heart"
219 - "Folsom Prison Blues"
221 - "All Hell Breaks Loose" pt 1
222 - "All Hell Breaks Loose" pt 2
Season 3
302 - "The Kids Are Alright"
303 - "Bad Day at Black Rock"
307 - "Fresh Blood"
308 - "A Very Supernatural Christmas"
310 - "Dream a Little Dream of Me"
311 - "Mystery Spot"
312 - "Jus in Bello"
315 - "Time Is On My Side"
316 - "No Rest for the Wicked"
Season 4
401 - "Lazarus Rising"
403 - "In the Beginning"
407 - "It's the Great Pumpkin, Sam Winchester"
409 - "I Know What You Did Last Summer"
410 - "Heaven and Hell"
415 - "Death Takes a Holiday"
416 - "On the Head of a Pin"
418 - "The Monster at the End of This Book"
419 - "Jump the Shark"
421 - "When the Levee Breaks"
422 - "Lucifer Rising"
Season 5
501 - "Sympathy for the Devil"
502 - "Good God, Y'All"
504 - "The End"
508 - "Changing Channels"
510 - "Abandon All Hope"
513 - "The Song Remains the Same"
514 - "My Bloody Valentine"
516 - "Dark Side of the Moon"
518 - "Point of No Return"
519 - "Hammer of the Gods"
521 - "Two Minutes to Midnight"
522 - "Swansong"
Season 6
601 - "Exile on Main Street"
603 - "The Third Man"
605 - "Live Free or Twi-Hard"
607 - "Family Matters"
610 - "Caged Heat"
611 - "Appointment in Samarra"
612 - "Like a Virgin"
616 - "And Then There Were None"
619 - "Mommie Dearest"
620 - "The Man Who Would Be King"
621 - "Let It Bleed"
622 - "The Man Who Knew Too Much"
Season 7
701 - "Meet the New Boss"
702 - "Hello Cruel World"
706 - "Slash Fiction"
709 - "How to Win Friends and Influence Monsters"
710 - "Death's Door"
715 - "Repo Man"
716 - "Out With the Old"
717 - "The Born Again Identity"
720 - "The Girl With the Dungeons and Dragons Tattoo"
721 - "Reading Is Fundamental"
722 - "There Will Be Blood"
723 - "Survival of the Fittest"
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