This is in no particular order, just randomly picked from the title gallery. Its also no coincidence that all but two of these episodes are from season 3 onward, that's where the show got really good imho.
1) Princess Cookie: I love this episode for two reasons. One, it can be seen as a tale of a cross dresser that defied society's rules to be himself and earn his happy ending. On the other, this aired with an episode of Regular Show where a one shot character kills himself and the day before that, the repeat of the first season finale of "The Legend of Korra" aired where there was a muder/suicide and the main character looked like SHE would kill herself. Most depressing days of cartoons I've ever seen.
2) Lady and Peebles: Season three onward I've begun to see Princess Bubblegum as somewhat of a villainous character. She treated her sucessor/son Lemongrab terribly in his first appearance, shows signs of being a mad scientist (her DNA may have made Goliad go evil), but this episode made me like her more, showing how badass she is. It was also pretty creepy too, which I love in this show.
3) Incendium: A very good episode that introduced the Fire Kingdom. I really like Flame King (voiced by the awesome Keith David) and Flame Princess. I did kind of feel lukewarm towards her as I thought she was basically Marceline combined with PB, but episodes with her in them made up for it.
4) The Eyes: A very funny episode with an unexpected twist. Especially the scene with Ice King is the season's first "Ice King is a lonely old man" card not really seen since "What is Life?"
5) Adventure Time with Fionna and Cake: Same as the above and the voices they picked for the gender counterparts were very well done. The closet one being Neil Patrick Harris doing the gender swapped Princess Bubblegum as Prince Gumball. I could hear Hayden Walch doing the same lines as NPH and it fit seemlessly.
6) Holly Jolly Secrets Part Two: This was a very good holiday episode with a very dramatic ending. Tom Kenny's monologue at the end was beautifully written and tear jerking.
7) I Remember You: One of the most definitive episodes of the series, beautifully made and perfect in every way. It makes you laugh, cry, smile and give insight of the mushroom war, the crux of the series.
8) Dungeon: A very fun episode in homage to the Legend of Zelda games, very enjoyable. The villain, a zombie cat creature voiced by Clancy Brown was great too.
9) Princess Monster Wife: A very great episode displaying that behind all the insanity, there lies a good man in the Ice King. A pretty creepy episode too, with him disecting various princesses to make up his frankenstein monster wife.
10) Goliad: I think my favorite episodes are either sad, creepy or funny or any mix in between. This was creepy (Anything Goliad does) and funny (Finn using mental blockades against Goliad's psychic attacks) and pretty creative too. And this is also evidence that something sinister lies in Bubblegum like my second pick above.
Inspired by this; [link]
Blank copy by: [link]
Title Cards provided by: [link]