For years fans had to sit through season after season, each filled with filler and non-filler episodes, confused which was important and which wasn't. Well I have gone through each past season , 1 - 9, and have done what wished someone would do: create an episode guide which takes out fillers and only has plot line importance episodes only. This is a 3 part project, each split up depending on region (Kanto & Orange Islands (1 - 2), Johto (3 - 5), Hoenn & Battle Frontier (6 - 9). So now I will continue to discuss this part.

Non-Filler Episode Guide: Sinnoh (Season 10 - ??)
Finish with Hoenn and the Kanto Battle Frontier, and his friends Brock, May, and Max gone their seperate ways, Ash heads off to Sinnoh with Pikachu on one shoulder and Aipom on the other. Ash will meet up with Brock once more and joining them is a new beginning coordinator named Dawn, they set out into the Sinnoh region for yet another series of adventures.

Key Guide:
Each episode is color-coded in order to inform you of it's importance. In some episodes a few important things happen, however the main one would be used as the color-code. Skippable episodes are optional to watch (but still sorta important).

Red - Region Starting/Ending Episode(s)
Brown - Part of Plotline
Blue - Main Character Introduced
Green - Main Pokemon Caught/Evolves/Revealed/Released
Gray - Character/Pokemon Development
Purple - Gym Battle
Yellow - Recurring Character Introduced
Pink - Pokemon Contest
* - Can be skipped

Pokemon Diamond & Pearl
DP001 "Following a Maiden's Voyage!"
DP002 "Two Degrees of Separation!"
DP003 "When Pokémon Worlds Collide!"
* DP004 "Dawn of a New Era!!"
DP005 "Gettin' Twiggy With It!"
* DP006 "Different Strokes for Different Blokes"
DP008 "Gymbaliar!"
DP009 "Setting the World on its Buneary!"
* DP010 "Not on MY Watch Ya Don't!"
DP011 "Mounting a Coordinator Assault!"
DP012 "Arrival of a Rival!"
DP013 "A Staravia Is Born!"
DP014 "Leave It To Brocko!"
DP015 "Shapes of Things to Come!"
DP016 "A Gruff Act to Follow!"
* DP017 "Wild in the Streets!"
DP018 "O'er the Rampardos We Watched!"
DP019 "Twice Smitten, Once Shy!"
* DP020 "Mutiny in the Bounty!"
* DP022 "Borrowing on Bad Faith!"
* DP025 "Oh Do You Know The Poffin Plan"
DP026 "Getting the Pre-Contest Titters"
DP027 "Settling a Not-So-Old Score!"
* DP029 "The Champ Twins!"
* DP030 "Some Enchanted Sweetening!"
DP031 "The Grass-Type is Always Greener!"
* DP032 "An Angry Combeenation!"
DP033 "All Dressed up with Somewhere to Go!"
DP034 "Buizel Your Way Out of This!"
* DP035 "An Elite Meet and Greet!"
DP036 "A Secret Sphere of Influence!"
DP037 "The Grass Menagerie!"
DP038 "One Big Happiny Family!"
* DP040 "Top-Down Training!"
* DP041 "A Stand-Up Sit-Down"
* DP044 "Mass Hip-Po-Sis!"
* DP045 "Ill-Will Hunting!"
* DP046 "A Maze-ing Race!"
DP046 "Dawn's Early Night"
DP050 "Tag! We're It!"
DP051 "Glory Blaze"
DP052 "Smells Like Team Spirit"

NOTE: If you think I missed any/added in an unnecessary/mis-colored an episode, please leave a comment of which episode it is. I will then look into it and make a response post to reveal what my investigations have resulted. If you did indeed find an error you will be credited for it. Happy episode watching!

CREDIT:
BAC510 - Pointing out DP046 "Dawn's Early Night" is a Contest episode.