I'll probably say Battling the Enemy Within http://bulbapedia.bulbagarden.net/wi...e_Enemy_Within

The very first episode (from the first series) or the gym battles with Lt. Surge

For each season, I always have an episode that I either record and never erase, or that I get on DVD/VHS. In order, starting with Season 1;
Ditto's Mysterious Mansion
Viva Las Lapras
Fowl Play
Imitation Confrontation
Johto Photo Finish
A Mudkip Mission
A Six-Pack Attack
Let it Snow, Let it Snow, Let it Snorunt
Numero Uno Articuno
Battling the Enemy Within (who DOESN'T like Dark Ash?)
O'er the Rampardos We Watched
One Team, Two Team, Red Team, Blue Team
Get Your Rotom Running (obviously, and it's also my all-time favorite)

The first episode (Pokémon, I Choose You!) is my all-time favorite. Other favorites include Go West, Young Meowth and Tears for Fears.

Pokemon Ranger and the Kidnapped Riolu - best multi-part episode
Chim-charred! - best single episode, just passing Ralts
Tears for Fears
(and all others involving Ash/Paul/Chimchar)
Crossing Paths
Pika and Goliath
(and nearly the entire 11th season)
Do I Hear a Ralts?
Disaster of Disguise/Disguise Da Limit
A Poached Ego
Address Unown
Talkin' 'Bout an Evolution/Rage Of Innocence
Turning Over a New Bayleef
Charizard Chills
Charmander, the Stray Pokemon
Ash Catches a Pokemon
Pokemon! I Choose You!

...and many more listed throughout my Pokemon site (just too many to list here)!

The episode in the first series (I think...) where they're in a haunted tower/mansion/etc. (I believe it's in Lavender), so that Ash can find a ghost Pokémon to fight Sabrina. I don't bother to remember the episode names ¬_¬
In the intro to the first series, one of the clips has a Haunter licking Charmander - That's the episode I'm on about.

EDIT: EP023 - The Tower of Terror

Haunter vs. Kadabra!! Mostly because it featured two of my then-favorite pokemon, but I also like the eeriness of Sabrina.

The episode where Pikachu was licking up the ketchup has always stuck in my mind. I can't remember the name, but I know it had to do with two Dojos fighting.

I also enjoyed the climax episodes of the Hoenn series (where Kyogre and Groudon fight, Lance comes in, the usual).

Island of the Giant Pokemon is my favorite episode.In the Pink and Islands of the Giant Pokemon & my favorite is Battling the Enemy Within because of dark Ash.

RageOfInnocence pretty much took all mine, Despite too many episodes in the Hoenn region.

Ok I like the VERY first episode, the one with Angie and her Shinx, when Ash battles Volkener (OMG LUXRAY), the one where because of Xatu, Ash and Dawn go back in time then come back (Palmer is CUTE!), and one of then new ones... awww can't remember what its called, anyways its the one with, Dialga, Palkia, Cyrus and the other commanders, Ash and friends, and Cynthia (Dialga and Palkia had a Arceus body halo on them!!!).

Ash hasn't battled Volkner yet.

He has in the Japanese series.

 And he won.

http://www.serebii.net/anime/epiguide/kanto/461.shtml
*purrs* Its like a weavile x sneasel kind of episode.<333 WEEEEAAA~♥
Please tell me you guys loved that episode. Cuz' pokemon really showed some kind of plot or conflict like humans do, I would TOTALLY watch the anime if there was some real conflict with pokemon, stories and many things instead of it all just being about humans and their adventures. Anyone agree?

For some reason, I'm drawn to the Indigo League episode when Ash first fought Sabrina.

My favorite episode is "Challenge of the samurai" and "Ash catches a Pokemon".

Probably the first episode, due to nostalgia. Shocking, isn't it?