Since his character's introduction to Supernatural, Castiel evolved. At first he was the angel who blindly followed orders. Then he questioned and doubted, resulting in becoming a temporary villain. Later he became human and then returned to his angel-state. Castiel learned to care about humans, becoming a father to Jack and family to the Winchesters.
Throughout his arc, with all its ups and downs, there are few things that stayed solid. One is his devotion to the Winchesters, especially Dean. Another is his desire to do good and help others. And yet another, is his sassy humor and quotes. Sometimes he meant to be humorous and sassy, and other times, he just naturally fell into it. While there are many, here are the top 10.
"No, He's Not On Any Flatbread"
In Season 5, Castiel decides to make his (and the Winchesters') mission to find God. Other than Michael, God is the powerful being who could stop Lucifer and the apocalypse. Since God isn't in heaven, Castiel reasons that God must be somewhere. He's almost giddy with the possibility of finding the long-lost God. Dean quips that Castiel should try New Mexico because God is rumored to be on a tortilla. Castiel responds without pause, "No, he's not on any flatbread."
Castiel may not have intended to deliver this sassy one-liner, but he did, and it remains an iconic line.
"I Learned That From the Pizza Man"
After Meg gets the blade from Castiel in order to save them from Crowley (when Crowley was the big-bad), Castiel passionately kisses her. The Winchester boys look on in confusion. Meg is more flustered than confused. She asks him where he learned to do that.
Little does she know that Castiel had been watching and almost studying porn in which the Pizza Man was the main character. He responds, "I learned that from the Pizza Man." Apparently, the Pizza Man was a great teacher as Castiel flustered a demon into becoming nearly speechless.
"Hey Ass Butt!"
Castiel is new to insults, but this is a valiant effort. To give Dean a chance with Sam (possessed by Lucifer), Castiel has to remove Michael. To get Michael's attention, he yells, "Hey Ass Butt," before hitting Michael with a bottle of holy fire. It's his attempt to give insults and be sassy.
It's definitely memorable, but not quite as kick-ass as he intended. Even Dean stops to look at him in the middle of his confrontation with Lucifer/Sam to question, "Ass butt?"
"At Least I Don't Look Like a Lumberjack"
In Season 12, the Winchester boys and Castiel go looking for Lucifer (who has now possessed a famous musician). When Castiel shows up to meet the boys in his iconic outfit, complete with trench coat, Dean remarks that his dress is too stiff for this town. Adding insult to injury, Dean says that if Castiel were considered an agent, it would only be third-tier.
Castiel responds back, "At least I don't look like a lumberjack," proving that he's mastered his sass and insult. By the look that Dean gives, the insult hit right where it should.
"Dean, You're Not a Talking Dog"
In the Scooby-Doo episode, Dean gets excited at the end once the mystery is solved. So, he looks into the camera and says, "Scooby Dooby Doo." Both Sam and Castiel look embarrassed, and Sam asks Dean what he's doing. Dean explains this is what always happens at the end of each episode when Scooby-Doo and the gang solve the mystery.
Castiel's response may be deadpan, but it's full sass as he says, "Dean, you're not a talking dog."
"I Found a Liquor Store, and I Drank It"
This episode from Season 5 has several sassy quotes from Castiel. Sam has been calling Castiel for help, and finally Castiel comes. However, Castiel isn't his normal self. As he stumbles into the room, Sam asks him what happened to him, and Castiel responds, "I found a liquor store, and I drank it."
Then, a concerned Sam asks if Castiel is okay. To which, Castiel responds, "Don't ask stupid questions." While this is an episode where fans see Castiel doubt his purpose and faith, it makes for some delicious sassy quotes.
"I Find the Sound of Your Voice Grating"
In the same Season 5 episode, Castiel gives viewers another zinger. When he comes to Sam, he enters the scene with complaints. Sam has been leaving Castiel messages. A drunk Castiel tells Sam that his message was long and then that he finds Sam's voice grating.
Of course, this is Castiel, drunk, but one wonders if there wasn't a gram of truth in it. In another season, Castiel didn't answer Sam's message, but come immediately when Dean prayed. Later, he explains to Sam that Dean and he have a more profound bond.
"My People Skills Are Rusty"
Castiel quickly arrives when Dean calls for him in Season 6, even though Sam has called for Castiel for a year. Maybe Castiel arrives because it was Dean, or maybe because angel weapons went topside and he's searching for them.
When he talks to the brothers, an irritated Castiel says, complete with air-quotes hand gestures, that, "my people skills are rusty. Pardon me, but I have spent the last year as a multidimensional wavelength of celestrial intent." This is Castiel at his sassiest, and it's a joy to see.
"I Believe the Hairless Ape Has the Floor"
Balthazar, Castiel's old friend and now soon-to-be foe, speaks to the Winchester brothers as if they are beneath him, an angel. Although he's been trapped by them with hellfire, he continues to insult them, calling them, "hairless apes."
Bathazar turns to Castiel for support, but Castiel angrily says, "I believe the hairless ape has the floor." His comment was one of respect to the Winchesters, but one of sass to Bathazar.
"Read the Bible. . .I'm Not Here to Perch On Your Shoulder"
In Season 4, Dean complains to Castiel about Castiel not helping them with the witnesses. Castiel explains that there were larger issues at hand and that heaven is in battle right now, that the angels can't constantly come to help when Dean calls. Dean says that he thought angels were guardians and not dicks. To which, Castiel responds, "Read the bible."
He expands, stating that he's a soldier and, "not here to perch on your shoulder." This is early Castiel, but the sass is still there and strong. It's a good example of all the sassy quotes that Castiel will deliver for the many seasons to come.