Verified answer

I'm not sure what you mean by "filler". There are episodes that are more tied to an overall story arc for the season and episodes that are more stand-alone or "Monster of the Week" (MOTW) episodes. An overall story arc became more important after season 2 so the first couple of seasons have more MOTW episodes. Although even with the MOTW episodes sometimes there's a scene that is tied to the story arc or they'll be an episode that is a MOTW for that season but introduce a character that become more important in later seasons/episodes. Plus some of the MOTW episode are the most fun - it would be a shame to miss them or think of them as only "filler".

So the episodes most tied with the overall story arcs would probably be

Season 1:

1.01 Pilot

1.05 Bloody Mary

1.11 Scarecrow

1.14 Nightmare

1.16 Shadow

1.20 Dead Man's Blood

1.21 Salvation

1.22 Devil's Trap

Season 2:

2.01 In My Time Of Dying

2.02 Everybody Loves a Clown

2.05 Simon Said

2.08 Crossroad Blues

2.09 Croatoan

2.13 Houses of the Holy

2.14 Born Under a Bad Sign

2.15 Tall Tales

2.20 What Is and What Should Never Be

2.21 All Hell Breaks Loose: Part One

2.22 All Hell Breaks Loose: Part Two

Season 3

3.01 The Magnificent Seven

3.02 The Kids Are Alright

3.04 Sin City

3.09 Malleus Maleficarum

3.10 Dream a Little Dream of Me

3.11 Mystery Spot

3.12 Jus in Bello

3.15 Time Is On My Side

3.16 No Rest For The Wicked

Season 4

4.01 Lazarus Rising

4.02 Are You There, God? It's Me, Dean Winchester

4.03 In The Beginning

4.04 Metamorphosis

4.07 It's The Great Pumpkin, Sam Winchester

4.09 I Know What You Did Last Summer

4.10 Heaven And Hell

4.14 Sex And Violence

4.15 Death Takes A Holiday

4.16 On The Head Of A Pin

4.17 It's A Terrible Life

4.18 The Monster At The End Of This Book

4.19 Jump The Shark

4.20 The Rapture

4.21 When The Levee Breaks

4.22 Lucifer Rising

Season 5

5.01 Sympathy For The Devil

5.02 Good God, Y'All

5.03 Free To Be You And Me

5.04 The End

5.08 Changing Channels

5.10 Abandon All Hope

5.13 The Song Remains The Same

5.14 My Bloody Valentine

5.15 Dead Men Don't Wear Plaid

5.16 Dark Side Of The Moon

5.17 99 Problems

5.18 Point Of No Return

5.19 Hammer Of The Gods

5.20 The Devil You Know

5.21 Two Minutes To Midnight

5.22 Swan Song

Season 6

6.01 Exile On Main St.

6.03 The Third Man

6.05 Live Free or Twihard

6.06 You Can't Handle The Truth

6.07 Family Matters

6.10 Caged Heat

6.11 Appointment in Samarra

6.12 Like A Virgin

6.13 Unforgiven

6.16 ...And Then There Were None

6.18 Frontierland

6.19 Mommy Dearest

6.20 The Man Who Would Be King

6.21 Let It Bleed

6.22 The Man Who Knew Too Much

But like I said, if you skip the other episodes you'll be missing some great stuff.

Episodes Of Supernatural

there is technically no filler because its not based off another work. there are some episodes that are plot centric and others that are less attached to the story at large. the other guy ^^^ has a good answer I suppose, but if you dont watch the full series you will be confused when there are kick backs to previous episodes and characters introduced when you havent seen them before but theve been involved in the series.

I suggest you watch the full series because it is awesome