Just 4 of the first 30 episodes were filler.
Challenge of the Samurai- Aside from Metapod evolving on the very end its mostly filler
Clefairy and the Moon Stone- Adapted from a game location, still filler
The Path to the Pokemon League- Pure Filler
The School of Hard Knocks- Pure Filler
Mystery at the Light House- Other than Ash capturing Krabby it's pretty much filler
Battle Aboard the St Anne- Filler
Pokemon Ship Wrecked- Filler
Island of the Giant Pokemon-Filler
Beauty and the Beach- PURE Filler
Tentacool and Tentacruel- Filler
Ghost At Maiden's Peak- PUREFiller
Bye Bye Butterfree- Its filler even if it gives a reason for Ash never using Butterfree again
Hypno's Naptime- Aside from Misty getting Psduck at the end its filler
Pokemon Fashion Flash- Aside from Brock geting Vulpix in the end its filler
Punchy Pokemon- It's filler
Sparks Fly for Magnemite- Filler
It would be easier to list the Kanto League episodes that aren't filler
Pokemon! I choose you- First episode
Pokemon! Emergency- First Team Rocket. Misty joins the party officially
Ash catches a Pokemon!- Conclusion of the series opener 3 parter.
Showdown in Pewter City- Boulder Badge, Brock introduce and joins
The Water Flowers of Cerulean Gym- Cascade Badge
Bulbasaur's Hidden Village- Ash gets one of his main Pokemon
Charmander the Stray Pokemon- See above
Here comes the Squirtle Squad- See above
Electric Shock Showdown- Thunder Badge
Abra and the Psychic Showdown- Marsh badge part 1
Tower of Tower- Marsh Badge part 2
Haunter vs Kadabra- Marsh Badge part 3
Ninja Poke Showdown- Soul Badge
March of the Exeggutor Squad- Mostly filler but important because Charmander evolves
Attack of the Prehistoric Pokemon- Charizard also Togepi egg
Who gets to Keep Togepi?- Togepi hatches
Riddle Me This- Volcano Badge part 1
Volcanic Panic- Volcano Badge part 2
Battle for the Badge- Earth Badge
All Fired Up- Arrival at the Indigo Plateau
Round One. Begin!- Ash registers. First match
Fourth Round Rumble- Gary gets knocked out. Ash's first real league battle (yes Fire and ice is filler it barely focuses on the league battles and you could treat his second and third battle as battles he won off screen and lose nothing)
A friend in deed- Introduce Ritchie
Friends and Foes alike- It's like 90 percent filler but its important in the end
Friends til the end- Concludes Indigo League